Holographic optical element and method of forming thereof

ABSTRACT

There is provided a holographic optical element including: a hologram portion including a plurality of groups of unit regions, each group of unit regions of the hologram portion being configured to produce a respective holographic image under a respective light illumination having a respective predetermined wavelength; and a colour filter portion formed on the hologram portion, the colour filter portion including a plurality of groups of unit regions, each group of unit regions of the colour filter portion being arranged on a corresponding group of the plurality of groups of unit regions of the hologram portion, whereby the plurality of groups of unit regions of the colour filter portion is spatially arranged to form a predetermined colour image. There is also provided a method of forming the holographic optical element. There is further provided an article having optical security incorporated therein.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a national phase application of PCT/SG2019/050213,filed on Apr. 15, 2019, which claims the benefit of priority ofSingapore Patent Application No. 10201803137R, filed 13 Apr. 2018, thecontent of which being hereby incorporated by reference in its entiretyfor all purposes.

TECHNICAL FIELD

The present invention generally relates to a holographic opticalelement, a method of forming thereof, and an article having opticalsecurity incorporated therein.

BACKGROUND

Optical security elements (e.g., optical security devices) are valuabletools in data encryption and document authentication as they exploitvarious properties of light, including amplitude, phase, polarisation,and wavelength, to create distinctive visual effects that can bedifficult to decode or duplicate. Two archetypal optical securitydevices are microscopic colour prints and holograms. Microscopic colourimages may be directly viewed under a magnifying glass, whereasholograms may be easily verified by using a laser pointer to project animage onto a screen placed in the far field (Fraunhofer regime). Tostrengthen the security of these basic devices, additional complexitymay be introduced by encoding multiple sets of information into a singledevice, i.e., multiplexing.

For example, multiplexed colour prints have been created by encodinginformation in two independent dimensions of elongated metalnanostructures, allowing for two different images to be read out underorthogonal polarisations of light. Using similar nanostructures ofvarious sizes optimised to respond to different wavelengths,three-colour multiplexed holograms based on the Pancharatnam-Berry (PB)geometric phase have also been demonstrated. Multiplexed PB hologramshave also been fabricated using an alternative geometry of nanoslits ina metal film. Unfortunately, transmission PB holograms often suffer fromlow transmission efficiency due to their use of lossy metalnanostructures and are complicated to read out, which may require theuse of circularly polarised light as well as specific illuminationand/or viewing angles. Additionally, the nanostructures are fabricatedwith electron beam lithography or focused ion beam milling, which incurshigh costs and imposes practical constraints on the patternable area.These shortcomings, namely low transmission efficiency, complexity ofreadout, and high fabrication costs, have limited their practicalapplication in optical security devices thus far.

SUMMARY

According to a first aspect of the present invention, there is provideda holographic optical element including:

-   -   a hologram portion including a plurality of groups of unit        regions, each group of unit regions of the hologram portion        being configured to produce a respective holographic image under        a respective light illumination having a respective        predetermined wavelength; and    -   a colour filter portion formed on the hologram portion, the        colour filter portion including a plurality of groups of unit        regions, each group of unit regions of the colour filter portion        being arranged on a corresponding group of the plurality of        groups of unit regions of the hologram portion,    -   wherein the plurality of groups of unit regions of the colour        filter portion is spatially arranged to form a predetermined        colour image.

According to a second aspect of the present invention, there is provideda method of forming a holographic optical element, the method including:

-   -   forming a hologram portion including a plurality of groups of        unit regions, each group of unit regions of the hologram portion        being configured to produce a respective holographic image under        a respective light illumination having a respective        predetermined wavelength; and    -   forming a colour filter portion on the hologram portion, the        colour filter portion including a plurality of groups of unit        regions, each group of unit regions of the colour filter portion        being arranged on a corresponding group of the plurality of        groups of unit regions of the hologram portion,    -   wherein the above-mentioned forming the colour filter portion        includes spatially arranging the plurality of groups of unit        regions of the colour filter portion to form a predetermined        colour image.

According to a third aspect of the present invention, there is providedan article having optical security incorporated therein, the articleincluding:

-   -   a substrate; and    -   one or more holographic optical elements according to the first        aspect of the present invention formed on the substrate for        providing the optical security.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be better understood andreadily apparent to one of ordinary skill in the art from the followingwritten description, by way of example only, and in conjunction with thedrawings, in which:

FIG. 1 depicts a schematic flow diagram of a method of forming aholographic optical element, according to various embodiments of thepresent invention;

FIGS. 2A and 2B depict schematic drawings of a holographic opticalelement, according to various embodiments of the present invention;

FIG. 3 depicts a schematic drawing of a top view of an article havingoptical security incorporated therein, according to various embodimentsof the present invention;

FIG. 4 depicts an exemplary holographic colour print, along with itscolour image and various holographic projections, according to variousexample embodiments of the present invention;

FIGS. 5A to 5E depict an example structure and example characteristicsof an exemplary holographic colour pixel, according to various exampleembodiments of the present invention;

FIGS. 6A to 6H relate to an exemplary two-tone holographic colour print,according to various example embodiments of the present invention;

FIGS. 7A to 7F relate to an exemplary six-colour holographic colourprint (corresponding to the Perfume print), according to various exampleembodiments of the present invention;

FIG. 8 depicts an exemplary method for designing or configuring anexample holographic colour print, according to various exampleembodiments of the present invention;

FIGS. 9A and 9B depict an exemplary method or process for fabricating anexemplary holographic colour pixel, according various exampleembodiments of the present invention;

FIGS. 10A to 10C depict exemplary ranges of colours attainable byvarying the pillar dimensions of the colour filters, according tovarious example embodiments of the present invention;

FIGS. 11A and 11B show comparisons of filter colours under differentconditions of block thickness, according to various example embodimentsof the present invention;

FIG. 12 depicts a table showing exemplary dimensions of pillars and thethickness-averaged RGB transmittances of the colour filters in thePerfume print shown in FIG. 7A, according to various example embodimentsof the present invention;

FIG. 13 depicts a table showing the RGB wavelength selectivity of thecolour filters in the Perfume print shown in FIG. 7A, according tovarious example embodiments of the present invention;

FIGS. 14A and 14B illustrate structural colour images of an exemplary QRcode viewed under different numerical aperture conditions, according tovarious example embodiments of the present invention;

FIG. 15 depicts photographs of exemplary Chinese seal holographicprojections captured at various illumination angles, according tovarious example embodiments of the present invention;

FIG. 16 illustrates the wide-angle characteristics of the exemplaryChinese seal holographic projection, according to various exampleembodiments of the present invention;

FIG. 17 illustrates the visibility of the Chinese seal holographicprojection under ambient lighting, according to various exampleembodiments of the present invention;

FIG. 18 illustrates simultaneous two-colour holographic projection fromthe exemplary QR code print, according to various example embodiments ofthe present invention;

FIG. 19 shows exemplary binary phase gratings made of phase plates andpillars, according to various example embodiments of the presentinvention;

FIG. 20 depicts a comparison of the diffracted power for phase plategratings and pillar array gratings, according to various exampleembodiments of the present invention;

FIG. 21 depicts a table showing the efficiency of the exemplary QR codeprint, according to various example embodiments of the presentinvention;

FIGS. 22A to 22C illustrate the effect of pixel arrangement onmultiplexed holograms, according to various example embodiments of thepresent invention; and

FIG. 23 shows the original source images and simulations, according tovarious example embodiments of the present invention.

DETAILED DESCRIPTION

In light of the foregoing, a need exists to provide a holographicoptical element, as well as a method of forming thereof, that seek toovercome, or at least ameliorate, one or more of the deficiencies orproblems associated with conventional holographic optical elements, suchas but not limited to, enhancing optical security and/or providingadditional functionality in an effective and/or efficient manner.

Various embodiments of the present invention provide a holographicoptical element, a method of forming thereof, and an article havingoptical security incorporated therein.

FIG. 1 depicts a schematic flow diagram of a method 100 of forming(e.g., fabricating or manufacturing) a holographic optical elementaccording to various embodiments of the present invention. The method100 includes forming (at 102) a hologram portion including a pluralityof groups of unit regions, each group of unit regions (i.e., each groupof a plurality of unit regions) of the hologram portion being configuredto produce a respective holographic image under a respective lightillumination having a respective predetermined wavelength; and forming(at 104) a colour filter portion on the hologram portion, the colourfilter portion including a plurality of groups of unit regions, eachgroup of unit regions of the colour filter portion being arranged on acorresponding group of the plurality of groups of unit regions of thehologram portion, whereby the above-mentioned forming (at 104) thecolour filter portion includes spatially arranging the plurality ofgroups of unit regions of the colour filter portion to form apredetermined colour image.

It will be understood by a person skilled in the art that a lightillumination having a predetermined wavelength include a lightillumination having only the predetermined wavelength or a range ofwavelengths including the predetermined wavelength. In other words, alight illumination having a predetermined wavelength is not limited to alight illumination having only the predetermined wavelength.

It will be understood by a person skilled in the art that theholographic optical element may be operable in various orientations andis not limited to any particular orientation(s). By way of an exampleonly and without limitation, in the case of the holographic opticalelement being positioned in a horizontal orientation, it will beunderstood by a person skilled in the art the colour filter portionbeing formed on the hologram portion may be either above (or over) thehologram portion (or on a top side thereof) or below (or under) thehologram portion (or on a bottom side thereof). By way of anotherexample only and without limitation, in the case of the holographicoptical element being positioned in a vertical orientation, it will beunderstood by a person skilled in the art the colour filter portionbeing formed on the hologram portion may be either to the left of thehologram portion (or on a left side thereof) or to the right of thehologram portion (or on a right side thereof).

In various embodiments, the holographic optical element may beconfigured as a transmission-type (i.e., a transmission holographicoptical element). In various other embodiments, the holographic opticalelement may be configured as a reflection-type (i.e., a reflectionholographic optical element).

In various embodiments, the predetermined colour image may be any colourimage desired or predetermined to be formed. In various embodiments, thepredetermined colour image may include two or more colours, three ormore colours, four or more colours, five or more colours, six or morecolours, and so on.

In various embodiments, the colour filter portion is configured toproduce the predetermined colour image independent of the differentholographic images produced by the hologram portion.

In various embodiments, each unit region of the hologram portion and itscorresponding unit region of the colour filter portion (i.e., the unitregion of the colour filter portion formed thereon) may form orconstitute a pixel (or pixel element or component) of the holographicoptical element. Therefore, the holographic optical element may includea plurality or an array of pixels, each pixel including a unit region ofthe hologram portion and the corresponding unit region of the colourfilter portion formed on the unit region of the hologram portion. Invarious embodiments, each unit region in the hologram portion and eachunit region in the colour filter portion may have the same planar sizeor dimensions (i.e., same dimensions (e.g., length (x) and width (y)) ina plane of the holographic optical element). In various examples, eachunit region in the colour filter portion may have the same square orrectangular planar dimensions. It will be understood by a person skilledin the art that the present invention is not limited to any specificplanar size or dimensions of the unit region, as well as shape or tilingthereof, and each unit region may be configured to have a planar size ordimensions as desired or as appropriate based on various factors, suchas a desired resolution of the colour image to be formed. In otherwords, each unit region may have planar dimensions configured based onthe desired resolution of the colour image to be formed. By way ofexample only and without limitation, each unit region may have planardimensions in a range of 0.5 μm to 10 μm, or preferably 1 μm to 5 μm, ormore preferably 2 μm to 3 μm, depending on various factors.

Accordingly, by forming the colour filter portion on the hologramportion such that each group of unit regions of the colour filterportion is arranged on a corresponding group of unit regions of thehologram portion, including spatially arranging (or configuring) theplurality of groups of unit regions of the colour filter portion to forma predetermined colour image, the plurality of groups of unit regions ofthe colour filter portion may advantageously be configured (e.g., basedon their spectral profile) to control passage of the holographicprojections (holographic images) by the plurality of groups of unitregions of the hologram portion, while at the same time, form a desiredcolour image. Therefore, the holographic optical element is not onlycapable of projecting different holographic projections under differentlight illuminations (e.g., different laser light), but is also capableof showing a desired colour image (e.g., a QR code, a particular logo ormark, a picture, a drawing or any selected or predetermined colourimage), thus advantageously enhancing optical security and/or providingadditional functionality in an effective and/or efficient manner.

In various embodiments, the above-mentioned forming (at 104) the colourfilter portion further includes interspersing at least one group of theplurality of groups of unit regions amongst one or more other groups(e.g. remaining groups) of the plurality of groups of unit regions. Inthis regard, such an interspersing amongst the plurality of groups ofunit regions may occur when the plurality of groups of unit regions ofthe colour filter portion is being spatially arranged to form thepredetermined colour image, for example, corresponding to the manner inwhich the plurality of colours (or colour groups) in the predeterminedcolour image may be interspersed amongst each other.

In various embodiments, controlling passage of the holographicprojections includes controlling transmission of the holographicprojections in the case of the holographic optical element being atransmission-type (i.e., a transmission holographic optical element)and/or controlling reflection of the holographic projection in the caseof the holographic optical element being a reflection-type (i.e., areflection holographic optical element).

In various embodiments, the above-mentioned forming (at 104) the colourfilter portion further includes configuring each group of unit regionsof the colour filter portion with wavelength selectivity (e.g.,wavelength selectivity of transmission or wavelength selectivity ofreflection) for the light illumination associated with the correspondinggroup of the plurality of groups of unit regions of the hologram portion(i.e., the light illumination having the predetermined wavelength basedon which the corresponding group of unit regions of the hologram portionis configured to produce the respective holographic image) and againstat least one of one or more light illuminations associated with one ormore remaining groups, respectively, of the plurality of groups of unitregions of the hologram portion. As a result, the group of unit regionsof the colour filter portion is able to hinder (e.g., minimize, divert,or block) at least one of the light illumination(s) associated with theremaining group(s) of the plurality of groups of unit regions of thehologram portion, thereby minimizing or preventing the holographicimage(s) associated with the remaining group(s) of the plurality ofgroups of unit regions of the hologram portion from being projected whenthe holographic image associated with the above-mentioned correspondinggroup of unit regions of the hologram portion is being projected. Forexample, this advantageously minimizes or prevents a “noisy” holographicimage being formed whereby unwanted holographic image(s) (e.g., ghostimage(s)), such as from other group(s) of unit regions of the hologramportion, weaken or distract the desired holographic image, such as fromthe desired group of unit regions of the hologram portion. As a result,different holographic images may be clearly produced by the holographicoptical element under different light illuminations having differentpredetermined wavelengths.

In various embodiments, each group of unit regions of the colour filterportion is configured with wavelength selectivity (e.g., wavelengthselectivity of transmission or wavelength selectivity of reflection) forthe light illumination associated with the corresponding group of theplurality of groups of unit regions of the hologram portion and againsteach of the one or more light illuminations associated with the one ormore remaining groups, respectively, of the plurality of groups of unitregions of the hologram portion.

In various embodiments, the plurality of groups of unit regions of thecolour filter portion are spatially arranged to form the colour imageunder a light illumination, such as a spatially coherent or incoherentlight illumination, a broadband light illumination or a white lightillumination.

In various embodiments, the above-mentioned forming (at 102) thehologram portion further includes: spatially arranging (e.g.,configuring) the plurality of groups of unit regions of the hologramportion to correspond to the spatial arrangement of the plurality ofgroups of unit regions of the colour filter portion forming thepredetermined colour image; and configuring each group of the pluralityof groups of unit regions of the hologram portion to produce therespective holographic image based on the spatial arrangement of thegroup of unit regions of the hologram portion.

In various embodiments, the above-mentioned forming (at 102) thehologram portion further includes: configuring respectively each unitregion of the group of unit regions of the hologram portion to have athickness for modifying a phase of the light illumination associatedtherewith (e.g., when transmitted therethrough or reflected therefrom)such that the group of unit regions of the hologram portion collectivelyproduce the respective holographic image under the light illuminationassociated therewith (i.e., the light illumination having thepredetermined wavelength based on which the above-mentioned group ofunit regions of the hologram portion is configured to produce therespective holographic image).

In various embodiments, the thickness of the hologram portion is in arange of about 0.6 μm to about 1.8 μm. For example, such a thicknessrange has been found to be preferred or optimal in the case of theholographic optical element being a transmission holographic print, aswill be described later below according to various example embodimentsof the present invention. In various embodiments, the range of thicknessvariation (i.e. the difference between the minimum and maximumthickness) may be a range from 0.1 μm to 10 μm, depending on variousfactors.

In various embodiments, the above-mentioned forming (at 104) the colourfilter portion further includes: configuring respectively each unitregion of the group of unit regions of the colour filter portion to havea spectral profile for allowing passage of the light illuminationassociated with the corresponding group of the plurality of groups ofunit regions of the hologram portion and for hindering passage of eachof the one or more light illuminations associated with the one or moreremaining groups, respectively, of the plurality of groups of unitregions of the hologram portion. In this regard, the above-mentionedconfiguring a group of unit regions with wavelength selectivity for alight illumination and against each of one or more other lightilluminations may include configuring each unit region of the group ofunit regions of the colour filter portion to have a spectral profile forallowing passage of the light illumination (having a predeterminedwavelength) and for hindering passage of each of the one or more otherlight illuminations (having different predetermined wavelengths,respectively).

In various embodiments, configuring a unit region of the colour filterportion to have a spectral profile for allowing passage of the lightillumination having a predetermined wavelength may include configuringthe unit region to have a spectral profile that allows passage of atleast 10% of the light illumination having the predetermined wavelength.In various embodiments, the spectral profile may be configured to allowat least 20%, at least 40%, at least 60%, at least 80% or 100% of thelight illumination having the predetermined wavelength.

In various embodiments, configuring a unit region of the colour filterportion to have a spectral profile for hindering passage of the lightillumination having a predetermined wavelength may include configuringthe unit region to have a spectral profile that hinders passage of atleast 50% of the light illumination having the predetermined wavelength.In various embodiments, the spectral profile may be configured to hinderat least 60%, at least 70%, at least 80%, at least 90% or 100% of thelight illumination having the predetermined wavelength. It will beunderstood to a person skilled in the art that the unit region may beconfigured to have a spectral profile that achieves any combination ofthe above-mentioned passage allowance percentage or percentage range andthe above-mentioned passage hindrance percentage or percentage range, asdesired or as appropriate.

In addition, it will be appreciated by a person skilled in the art thatfor a given spectral profile of a colour filter, the wavelength(s) ofallowance and the wavelength(s) of hindrance are different, and thus forexample the percentage (or percentage range) of light passage at thevarious wavelengths of interest need not (and in practice may not) sumto 100%. For example, it will be appreciated by a person skilled in theart that the relative percentage of light passage (e.g., wavelengthselectivity), instead of solely the absolute percentage, may be ofinterest as it may be the relative strength of the desired and unwanted(crosstalk) holographic images that determines how noisy the resultantprojected holographic images appear. By way of an example only andwithout limitation, referring to FIG. 6B (which will be described laterbelow), the wavelength selectivity for red light may be the ratio of thered light transmittance of the yellow colour filter over that of theblue colour filter. In this regard, a larger ratio may be better, andmathematically the ratio may be more strongly improved by decreasing thered transmittance of the blue filter than by increasing the redtransmittance of the yellow filter. In other words, maximally hinderingthe passage of unwanted light illumination may be more important thanmaximally allowing the passage of desired light illumination.

As a further example, consider a hypothetical extreme case in which thecolour filters have a perfect hindrance of 100% at unwanted wavelengths,and as a result there is no crosstalk from unwanted holographic images.Then as long as even a small amount of light is able to pass at thedesired wavelengths (e.g., 1% light passage allowance), the opticalelement may have infinite wavelength selectivity and may give excellentperformance with low noise. Note also that the low light passageallowance may be compensated by using high power light illumination. Onthe other hand, consider a case in which the colour filters have a lightpassage hindrance of 50% at unwanted wavelengths, and as a result theremay be relatively strong crosstalk from unwanted holographic images. Inthis case, even with a perfect 100% light passage allowance at thedesired wavelengths, the desired holographic images may be at most twiceas strong as the unwanted holographic images (i.e., wavelengthselectivity of 2), which may impose a limit on the performance of theholographic optical element. In this regard, changing the lightillumination power may not help as it does not affect the relativestrength of the holographic images.

Accordingly, in the context of holographic optical elements according tovarious example embodiments, it may be reasonable for colour filterswith even a small light passage allowance at their desired wavelength(s)(by way of an example only and without limitation, 10%) to produce asatisfactory outcome if the light passage hindrance of the unwantedwavelength(s) is sufficiently complete (by way of an example only andwithout limitation, 98%, i.e., a light passage allowance of 2%). Forexample, in the case of 10% desired passage vs 2% unwanted passage, theunwanted holographic images would be 5 times weaker than the desiredholographic image, leaving the desired holographic image relativelyundisturbed.

Accordingly, each group of unit regions of the colour filter portion maybe configured with wavelength selectivity for a desired lightillumination and against other undesired light illumination(s) based onvarious factors as described above, such as in a manner for sufficientlyachieving minimal or zero (or non-observable) crosstalk from unwantedholographic images. In various embodiments, the wavelength selectivitymay have a ratio of passage at selected wavelength(s) over otherselected wavelength(s) (e.g., desired wavelength(s) overunwanted/undesired wavelength(s)) of 2 or more, or preferably 3 or more,4 or more, 5 or more, 7 or more, 10 or more and so on. For betterunderstanding, by way of examples only and without limitation, examplewavelength selectivity ratios will be described later below withreference to FIG. 13, according to various example embodiments.

In various embodiments, the above-mentioned forming (at 104) the colourfilter portion further includes forming each unit region of the group ofunit regions of the colour filter to include an array of pillarstructures (or elongated or protruding structures).

In various embodiments, the hologram portion and the colour filterportion are made of a dielectric material.

In various embodiments, the hologram portion and the colour filterportion are formed as a monolithic structure, for example, an integratedstructure made from a single material.

In various embodiments, the light illuminations associated with theplurality of groups of unit regions of the hologram portion,respectively, are laser illuminations (or laser light) and are differentfrom each other (e.g., each being selected from a differentnon-overlapping wavelength or range of wavelengths (or wavelength band).

In various embodiments, the laser illuminations associated with theplurality of groups of unit regions of the hologram portion are eachselected from a group consisting of a red laser illumination (e.g., 600nm to 700 nm), a green laser illumination (e.g., 500 nm to 600 nm) and ablue laser illumination (e.g., 400 nm to 500 nm).

In various embodiments, the light illuminations may have a wavelength(or range of wavelengths) outside the visible spectrum (non-visiblelight spectrum), such as infrared radiation (IR) (or infrared light)(e.g., 700 nm to 900 nm) and ultraviolet (UV) light (e.g., 200 nm to 400nm).

In various embodiments, each laser illumination may be selected from avisible spectrum and non-visible light spectrum, such as those mentionedabove. In various embodiments, the choice or selection of colours may bedetermined based on the choice or selection of materials and/or geometryof the colour filter.

FIG. 2A depicts a schematic drawing of a side view of a holographicoptical element 200 according to various embodiments of the presentinvention, and FIG. 2B depicts schematic drawings of a top view of ahologram portion 202 and a colour filter portion 204. The holographicoptical element 200 includes: a hologram portion 202 including aplurality of groups 205/206 of unit regions 208, each group 205/206 ofunit regions 208 of the hologram portion 202 being configured to producea respective holographic image (not shown in FIGS. 2A and 2B) under arespective light illumination having a respective predeterminedwavelength; and a colour filter portion 204 formed on the hologramportion 202, the colour filter portion 204 including a plurality ofgroups 215/216 of unit regions 218, each group 215/216 of unit regions218 of the colour filter portion 204 being arranged on a correspondinggroup 205/206 of unit regions 208 of the hologram portion 202, wherebythe plurality of groups of unit regions of the colour filter portion isspatially arranged to form a predetermined colour image.

In FIG. 2A, for illustration purposes only and without limitation, thehologram portion 202 and the colour filter portion 204 are shown as twolayers. However, it will be understood by a person skilled in the artthat the hologram portion 202 and the colour filter portion 204 may notbe separate layers, and may together form a monolithic structure, forexample, an integrated structure made from a single material, asdescribed herein according to various embodiments.

In FIG. 2B, for illustration purposes only and without limitation, unitregions belonging to a particular group of unit regions are denoted bythe same pattern or shading. For example, in FIG. 2B, the hologramportion 202 includes a first group 205 of unit regions denoted by thesame diagonal stripe pattern, and a second group 206 of unit regionsdenoted by the same horizontal stripe pattern. Similarly, the colourfilter portion 204 includes a first group 215 of unit regions denoted bythe same diagonal stripe pattern, and a second group 216 of unit regionsdenoted by the same horizontal stripe pattern. Furthermore,corresponding groups of unit regions of the hologram portion 202 and thecolour filter portion 204 are also denoted by the same pattern. It willbe appreciated by a person skilled in the art that the present inventionis not limited to the number of unit regions or number of groups of unitregions (or the size of the hologram portion 202 and/or the colourfilter portion 204) as shown in FIG. 2B, and any number of unit regionsor number of groups of unit regions may be provided or formed as desiredor as appropriate depending on various factors, such as the size of thecolour image desired to be formed. It will also be appreciated by aperson skilled in the art that the configuration (or arrangement) of thegroups of unit regions shown in FIG. 2B is simply an arbitraryconfiguration for illustration purposes only and the present inventionis not limited thereto. The groups of unit regions may be configured (orarranged) as desired or as appropriate based on various factors, such asthe colour image and/or the holographic images desired to be formed.Exemplary methods for configuring the groups of unit regions of thehologram portion 202 and the colour filter portion 204 will be describedlater according to various example embodiments of the present invention.

In various embodiments, the holographic optical element 200 correspondsto the holographic optical element formed by the method 100 as describedhereinbefore with reference to FIG. 1, therefore, various features ofthe holographic optical element 200 may correspond to (e.g., the sameas) those of the holographic optical element formed by the method 100according to various embodiments, and thus need not be repeated withrespect to the holographic optical element 200 shown in FIGS. 2A and 2Bfor clarity and conciseness. In other words, various embodimentsdescribed herein in context of the method 100 as shown in FIG. 1 areanalogously valid for the holographic optical element 200 shown in FIGS.2A and 2B, and vice versa.

FIG. 3 depicts a schematic drawing of a top view of an article 300having optical security incorporated therein (e.g., embedded therein,attached thereto or applied thereto), according to various embodimentsof the present invention. The article 300 includes a substrate 302 andone or more holographic optical elements 200 as described hereinaccording to various embodiments, such as with reference to FIGS. 1, 2Aor 2B. It will be appreciated by a person skilled in the art that anynumber of holographic optical elements 200 may be incorporated in anyarticle of the interest desired to have optical security (e.g., foranti-counterfeiting protection), for example, various items and/ordocuments, such as but not limited to, smart cards, banknotes, securitydocuments (e.g., identity card or passport), certificates, luxury orhigh-value consumer products and so on.

It will be appreciated by a person skilled in the art that theterminology used herein is for the purpose of describing variousembodiments only and is not intended to be limiting of the presentinvention. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“includes” and/or “including,” or the like (e.g., “includes” and/or“including”) when used in this specification, specify the presence ofstated features, integers, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof.

In order that the present invention may be readily understood and putinto practical effect, various example embodiments of the presentinventions will be described hereinafter by way of examples only and notlimitations. It will be appreciated by a person skilled in the art thatthe present invention may, however, be embodied in various differentforms and should not be construed as limited to the example embodimentsset forth hereinafter. Rather, these example embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the present invention to those skilled in the art.

In particular, for better understanding of the present invention andwithout limitation or loss of generality, various example embodiments ofthe present invention will now be described with respect to aholographic optical element being in the form of a transmissionholographic colour print (or more specifically, a transmissionmultiplexed holographic colour print, or may also simply be referred toas a “print” herein) for optical security.

Conventional optical security devices provide authentication bymanipulating a specific property of light to produce a distinctiveoptical signature. For instance, microscopic colour prints modulate theamplitude, whereas holograms typically modulate the phase of light.However, their relatively simple structure and behaviour may be easilyimitated. In this regard, various example embodiments provide a pixelthat overlays a structural colour element (e.g., corresponding to the“unit region” of the colour filter portion as described herein accordingto various embodiments) onto a phase plate (e.g., corresponding to the“unit region” of the hologram portion as described herein according tovarious embodiments) to control both the phase and amplitude of light,and arrays these pixels into a monolithic print that exhibits complexbehaviour. In this manner, the fabricated holographic colour printaccording to various example embodiments appears as a colour image underwhite light, while projecting up to, for example, three differentholograms under, for example, red, green and blue laser illuminations,respectively. The holographic colour print can be readily verified butchallenging to emulate, and can provide enhanced security inanti-counterfeiting applications. In various example embodiments, as theholographic colour print encodes information only in the surface reliefof a single polymeric material, nanoscale 3D printing of customisedmasters may enable their mass-manufacture by nanoimprint lithography.

As described in the background, optical security elements (e.g., opticalsecurity devices) are valuable tools in data encryption and documentauthentication as they exploit various properties of light, includingamplitude, phase, polarisation, and wavelength, to create distinctivevisual effects that can be difficult to decode or duplicate. Twoarchetypal optical security devices are microscopic colour prints andholograms. Microscopic colour images may be directly viewed under amagnifying glass, whereas holograms may be easily verified by using alaser pointer to project an image onto a screen placed in the far field(Fraunhofer regime). To strengthen the security of these basic devices,additional complexity may be introduced by encoding multiple sets ofinformation into a single device, i.e., multiplexing.

For example, multiplexed colour prints have been created by encodinginformation in two independent dimensions of elongated metalnanostructures, allowing for two different images to be read out underorthogonal polarisations of light. Using similar nanostructures ofvarious sizes optimised to respond to different wavelengths,three-colour multiplexed holograms based on the Pancharatnam-Berry (PB)geometric phase have also been demonstrated. Multiplexed PB hologramshave also been fabricated using an alternative geometry of nanoslits ina metal film. Unfortunately, transmission PB holograms often suffer fromlow transmission efficiency due to their use of lossy metalnanostructures and are complicated to read out, which may require theuse of circularly polarised light as well as specific illuminationand/or viewing angles. Additionally, the nanostructures are fabricatedwith electron beam lithography or focused ion beam milling, which incurshigh costs and imposes practical constraints on the patternable area.These shortcomings, namely low transmission efficiency, complexity ofreadout, and high fabrication costs, have limited their practicalapplication in optical security devices thus far.

Various example embodiments note that, in comparison, traditional phaseelements including dielectric structures of different thicknesses (phaseplates) enable holographic projection to be achieved with highertransmission efficiency, simpler illumination methods (e.g. a handheldlaser pointer), and little restriction on the polarisation or incidenceangle of the light. They are also potentially easier and cheaper tomanufacture than PB nanostructures as their larger dimensions are withinthe resolution limit of photolithography. High transmission efficiencymultiplexed holograms that project up to three different imagesdepending on the incident wavelength have previously been demonstratedusing a variety of techniques including phase modulation and depthdivision. Recently, white-light transmission colour holograms operatingin the Fresnel limit have also been developed.

However, various example embodiments note that as phase hologramsconventionally are not designed to control the amplitude of light, theygenerally appear random or featureless under incoherent illumination,which make them less attractive or less suitable as optical securitydevices. Conversely, colour images have superior decorative values onbanknotes or documents but generally do not produce any meaningfulholographic projection under coherent illumination as they generally donot control the phase of the light. In this regard, various exampleembodiments identified that introducing a design methodology to controlthe phase and amplitude of light simultaneously is an area that has beenrelatively unexplored and may enable the creation of a dual-functiondevice that appears as an image in plain view, but encrypts additionaldata that can be retrieved through holographic projection. In variousexample embodiments, such an encryption refers to the fact that theholographic information cannot be read except by illumination withcoherent monochromatic light of a suitable wavelength, noting that theintroduction of one or more additional phase masks as security keys mayenable even more secure encryption.

Accordingly, various example embodiments provide an optical securityelement (e.g., optical security device) that combines phase andamplitude control to integrate holograms (i.e., multiple holograms) intoa colour print, which may herein be referred to as a “holographic colourprint” (or more specifically, a “multiplexed holographic colour print”,or may also simply be referred to as a “print”). For example, theoptical security device may appear as a colour image when viewed inwhite light, but may reveal up to, for example, three different hiddengrayscale holographic projections under, for example, red, green, andblue laser illuminations, respectively. Accordingly, in various exampleembodiments, the multiple holograms are advantageously encrypted into acolour print. Integrating multiple holograms into a colour print mayrequire the ability to encode both phase and colour independently withinindividual pixel elements, which for example is a challenge for recentPB holograms. The holographic colour print according to various exampleembodiments provides a unique and easily recognisable visual effect, andmay be applied in, for example, the security industry as effectiveanti-counterfeiting elements that provide enhanced optical security onimportant documents, such as identity cards and passports.

FIG. 4 depicts an exemplary holographic colour print 400, along with itscolour image 420 and various holographic projections 422 a, 422 b, 422c, according to various example embodiments of the present invention. Inparticular, FIG. 4 depicts an exploded-view schematic of the holographiccolour print 400 in the form of a layered optical device in which colourfilters 404 are integrated on top of holograms 402. The colour filters404 may act as colour pixels in a colour image 420 under white lightillumination, such as from a lamp or torch, and also serve to controlthe transmission of red, green and blue (RGB) laser light through thepixels of the underlying multiplexed holograms 402, as illustrated inFIG. 4. For example, under RGB laser illumination, each wavelength oflight may select a different holographic projection 422 a, 422 b, 422 c(e.g., corresponding to the “respective holographic image” describedherein according to various embodiments), which is independent of thecolour image 420 and the other projections. Far field projections appearalong the axis of laser illumination. In FIG. 4, three different anglesof incidence are shown to illustrate the three distinct holographicprojections. The projections remain in focus at any distance in the farfield, can be achieved over a wide range of incident angles, and areperfectly overlapped under collinear multi-colour illumination.

Accordingly, a working or operating example of a transmissiveholographic colour print 400 is illustrated in FIG. 4. The holographiccolour print 400 may include a top layer (or a first layer) 404 (e.g.,corresponding to the “colour filter portion” described herein accordingto various embodiments) including colour filters that encode a colourprint, and a bottom layer (or a second layer) 402 (e.g., correspondingto the “hologram portion” described herein according to variousembodiments) including phase plates that encode the holograms. Thecolour filters 404 may be configured to have two functions: (1) tocollectively form a colour image 420 under white light illumination, and(2) to control the transmission of red, green and blue (RGB) laser lightthrough the pixels of the underlying multiplexed holograms. Withcoherent monochromatic illumination (e.g. light from a laser pointer),the incident light may be filtered such that only the relevant phaseplates with colour filters that closely match the illuminationwavelength are selected for a given holographic projection, whereas theother phase plates do not form a projection as their colour filters aremismatched and reject the incident light. Therefore, the multiplexedholographic colour print 400 is operable to show different holographicprojections 422 a, 422 b, 422 c when illuminated by red, green and bluelasers, respectively.

Accordingly, because incident light passes through all pixels inparallel, the pixels may act independently to allow transmission ofdifferent wavelengths in some regions of space (e.g., a group of pixelsof the holographic colour print 400 corresponding a group of unitregions of the hologram portion 402 configured to produce a particularholographic image) but not others (e.g., other group(s) of pixels of theholographic colour print 400 corresponding to other group(s) of unitregions of the hologram portion 402 configured to produce otherholographic image(s)), which enables several holograms to jointly occupythe total area available of the holographic colour print 400 based on aspatial multiplexing technique. Using the freedom to divide the spaceinto regions of arbitrary shapes and sizes (e.g., various groups ofpixels of the holographic colour print 400 forming various shapes andsizes), the individual hologram areas (e.g., the plurality of groups ofunit regions of the hologram portion 402) may then be strategicallyallocated or configured such that the corresponding arrangement of thecolour filters additionally encodes a chosen or desired colour image420. Under incoherent white light illumination (e.g., light from a lampor torch), the phase modulation of the holograms may be effectivelyignored and the colour filters may act as amplitude-modulating colourpixels that together show the desired colour image 420.

Exemplary design(s)/configuration(s) of a holographic colour pixel willnow be described according various example embodiments of the presentinvention.

In various example embodiments, to create a physical realisation of aholographic colour print, a holographic colour pixel may first be formedor developed that controls both the phase and amplitude of light. Invarious example embodiments, the pixel may have a relatively largeminimum feature size of several hundreds of nanometres and includes(e.g., entirely made of) a single dielectric material. As a result, theholographic colour print is able to be fabricated using femtosecond 3Dprinting (direct laser writing) as a monolithic structure in across-linked polymer. For illustration purposes, an example fabricationprocess will be described later below according to various exampleembodiments of the present invention.

FIGS. 5A to 5E depicts an example structure and example characteristicsof an exemplary holographic colour pixel 500 according to variousexample embodiments of the present invention. In particular, FIG. 5Adepicts a schematic drawing of a structure of the holographic colourpixel 500 that is configured to provide combined phase and amplitudecontrol, which includes an array 502 of pillars 506 (e.g., correspondingto “colour filter” described herein) integrated on top of a block 504(e.g., corresponding to a “phase plate” described herein) made of adielectric material with refractive index n. The colour filter 502 maybe configured to control the amplitude of light based on itstransmission spectrum T(λ)=f(h, d, p), which depends on the pillar arraydimensions, for example, {h, d, p} (height, diameter and pitch). Thephase plate 504 may be configured to control the phase of transmittedlight where the phase shift arises from path length differences thatdepend on the block thickness.

FIG. 5B depicts the transmission spectra and corresponding opticalmicrographs of pillar arrays with red, green and blue colours,respectively. The transmission spectra were averaged from measurementsof the pillar arrays with the same dimensions as those shown in FIGS. 5Cto 5E, but patterned separately on blocks of uniform thickness (0.6 μm,1.0 μm, 1.4 μm, 1.8 μm). Good RGB wavelength selectivity can be seenfrom a high transmittance (average 62%) for red (638 nm), green (527nm), and blue (449 nm) for light passing through their respectivefilters (filled circles), and low transmittance (average 15%) for lightpassing through the wrong filters (empty circles).

FIGS. 5C to 5E depict the false-colour tilt-view SEMS of pillar arrayswith dimensions optimised to give the best or optimal selectivity forred, green and blue light, according to various example embodiments ofthe present invention. By way of an example and without limitations,according to various example embodiments, the pillars (about 0.4 μm indiameter and respectively 1.9 μm, 0.7 μm and 2.6 μm in height) may bepatterned in a square array of 1 μm pitch onto 3×3 μm² blocks ofrandomly varying thickness within a thickness range to be used forhologram phase plates (thickness range of 0.6 μm to 1.8 μm). In FIGS. 5Cto 5E, the scale bars each denote 2 μm.

Accordingly, the pixel design or configuration according to variousexample embodiments integrates a dielectric phase plate 504 under astructural colour element 502 including an array of dielectric pillars506, which acts as a colour pixel for the transmission colour imageunder white light illumination and also as a colour filter toselectively transmit red, green, or blue laser light for hologrammultiplexing. The colour filters 502 are configured to be diffractive,thereby transmitting the desired wavelengths of light on-axis andrejecting unwanted wavelengths by diffracting them off-axis at largeangles (e.g., see FIG. 16, which will be described later below).According to various example embodiments, the dielectric phase plate 504controls the phase of transmitted light according to the equationϕ(λ)=2π(n−1)t/λ, where the phase shift (ϕ) arises from path lengthdifferences that depend on the phase plate thickness (t) and refractiveindex (n). The refractive index of the dielectric polymer material used(e.g., between 1.54 and 1.58 in the visible region) according to variousexample embodiments has been found to allow a full 2π phase modulationto be achieved for red, green and blue lights by varying the phase platethickness over a range of 1.2 μm. In various example embodiments, as itmay be impractical to fabricate samples with a continuous phasevariation, the phase is quantised and a number of discrete thicknesssteps are defined to span the required or desired 1.2 μm range (e.g.,see Section on “Phase Plate Thickness Calibration”, which will bedescribed later below).

Assuming that a phase plate acts as an ideal phase-controlling(constant-amplitude) element and a pillar array colour filter acts as anideal amplitude-controlling (constant-phase) element, various exampleembodiments combine these elements into a layered pixel for independentphase and amplitude control. In practice, however, various exampleembodiments identified that due to the refractive index mismatch betweenthe glass substrate and the polymer structures, changing the thicknessof the underlying block to control the phase may affected thetransmission amplitude of the overall pixel, i.e., phase-amplitudecoupling may be present. In this regard, various example embodimentsfound that the shift in transmission spectrum with block thicknesscaused a significant change in the pixel colour with thin blocks of t=0to 0.4 μm, but was not noticeable for thicker blocks of t≥0.6 μm (e.g.,see FIGS. 10A to 10C, which will be described later below). As such,various example embodiments configures blocks having a thickness in arange of 0.6 μm to 1.8 μm thickness to span the required range of 1.2μm.

To minimise any remaining variations in pixel transmission amplitude dueto differences in phase plate height, various example embodimentsfabricate dielectric pillar arrays on blocks with thicknesses varyingbetween 0.6 to 1.8 μm and measure their transmission spectra T(λ). Thedependence of T(λ) on thickness may then be averaged out, effectivelyeliminating any residual thickness dependence. Subsequently, the pillararray dimensions of height (h), diameter (d), and pitch (p) may bevaried or configured to enable access to a range of colours spanninggreater than 50% of the sRGB colour gamut (e.g., see FIGS. 11A and 11B,which will be described later below), from which the most suitablefilters for red, green and blue wavelengths may then be selected (e.g.,see Section on “Colour Palette and Wavelength Selectivity”, which willbe described later below). The transmission spectra of the RGB colourfilters (e.g., see FIG. 5B) with optimised pillar array dimensions(e.g., see FIGS. 5C to 5E) show a high transmittance averaging 62% atthe desired wavelength and a low transmittance averaging 15% at unwantedwavelengths (e.g., see Table 1200 shown in FIG. 12, which will bedescribed later below). These transmittance values afford sufficientwavelength selectivity, i.e., mutually exclusive or orthogonaltransmission, at the wavelengths of interest (e.g., see Table 1300 shownin FIG. 13, which will be described later below). For example, the widecolour range enables the ability to choose or select suitable colours toreproduce the colour image under white light illumination, and the goodwavelength selectivity ensures that laser light can be filtered todistinguish the individual holograms.

In addition to reducing the effects of pixel phase on pixel amplitude(phase-amplitude coupling), which may otherwise affect multiplexing andcolour image formation, various example embodiments further investigatedthe effect of pixel amplitude on pixel phase (amplitude-phase coupling),which may affect the holographic projections. It was found that thepillar colour filters may add a weak unwanted phase variation on top ofthe desired phase variation controlled by the phase plate thickness(e.g., see FIGS. 19 and 20, which will be described later below).However, while this extra phase can be compensated for at the pixellevel if necessary or desired, it was found that the extra phase was anorder of magnitude smaller than the effect of the phase plates (e.g.,see Section on “Measuring Amplitude-Phase Coupling”, which will bedescribed later below). Accordingly, various example embodiments safelyneglect the extra phase in the holographic colour prints.

Exemplary method(s) for fabrication of holographic colour prints willnow be described according various example embodiments of the presentinvention.

FIGS. 6A to 6H relate to an exemplary two-tone holographic colour print600 according to various example embodiments of the present invention.FIG. 6A illustrates a design technique for the holographic colour print600, including the ability to choose the amplitudes of the pixelsindependent of their phase makes it possible to rearrange the pixels ina multiplexed hologram (spatial freedom) so that the pixel arrangementincludes meaningful information as well, allowing a colour image to beshown on top of the holograms. In various example embodiments, as longas the phase is calculated taking the pixel arrangement into account,the individual holographic projections can still be maintained. FIG. 6Bdepicts a transmission spectra of the yellow and blue colour filtersused, which have mutually exclusive (orthogonal) transmission at thewavelengths of interest (638 nm red and 449 nm blue). FIG. 6C depicts anoptical characterisation of the holographic colour print 600. Inparticular, FIG. 6C depicts a transmission optical micrograph of thetwo-tone multiplexed hologram 600 in which the pixels are arranged toform a 480×480 pixel colour QR code (e.g., 1.44 mm²). The blue colourfilters (blue hologram channel) 602 selectively pass blue light but notred light, whereas the yellow colour filters (red hologram channel) 604selectively pass red light but not blue light. FIGS. 6D and 6E depictholographic projections in transmission, photographed on a white wall ina darkened room. In particular, FIG. 6D depicts an image of a Chineseseal shown under 638 nm red laser illumination and FIG. 6E depicts animage of a Penny Black stamp shown under 449 nm blue laser illumination.The projection size scales with the projection distance, reaching anaverage size of approximately 10 cm at a distance of 1 m. FIGS. 6F to 6Hdepict scanning electron micrographs (SEMs) of the holographic colourprint 600 at various scales, namely, 200 μm, 20 μm and 5 μm,respectively. Each holographic colour pixel includes a 3×3 pillar arraycolour filter on top of a 3×3 μm² phase plate, and each QR code pixel isa super-pixel including a 4×4 block of holographic colour pixels. In theclose-up tilt-view SEM, a blue and a yellow QR code super-pixel arehighlighted in false colour, and the bottom-right corner holographiccolour pixel of each is further highlighted.

Accordingly, based on the holographic colour pixel design/configurationas described hereinbefore, various example embodiments are able tocreate or form multiplexed holograms by fabricating large arrays ofpixels. In an example simplest case, holograms may be multiplexedside-by-side with the phase plates of each hologram spatially segregatedin contiguous single-coloured regions, giving a similar result to thatachievable by pasting macroscopic colour filters onto a spatial lightmodulator. However, such multiplexing technique cannot be used torealise an arbitrary or desired colour image. In contrast, in thedesign/configuration of the holographic colour prints according tovarious example embodiments, the ability to control phase and amplitudeon the level of individual pixels enables the freedom to move pixelsaround as illustrated in FIG. 6A as long as the phase is recalculatedfor any new pixel arrangement. As the total area allocated to eachhologram is not fixed, pixels of one hologram may be replaced withpixels of another hologram as long as each hologram still transmitsenough light to give a reasonably high signal-to-noise ratio (e.g., seesection on “Balancing Wavelength Selectivity in Multiplexed Holograms”,which will be described later below). Having the option to freelyrearrange and replace pixels (“spatial freedom”) generally provides theability to choose any arbitrary or desired pixel arrangement. Variousexample embodiments identified a few factors or exceptions, which willbe described later below under section “Pixel Arrangement for SpatialMultiplexing of Holograms”. Advantageously, it was found that replacingpixels, rearranging amplitudes, and recalculating the phases does notgreatly affect the fidelity of holographic projection when the number ofpixels is sufficiently large (e.g. 480×480 pixels, as used in variousexample embodiments).

For simplicity, the multiplexing of two holograms into a two-tone image600, being a QR code, is first demonstrated according to various exampleembodiments. Various example embodiments note that although theholograms in this example are designed for red and blue laserillumination, there is no need for the colour filters used formultiplexing to be red and blue as long as their transmission amplitudesare mutually exclusive (orthogonal) at the design wavelengths. Becausethe wavelength selectivity requirement constrains the spectra at onlytwo points, there are a number of possible spectra and therefore coloursthat can be used. As such, there exists some flexibility to choosecolour filters with transmission spectra that best match both thedesired image colours and the hologram design wavelengths (“spectralfreedom”), or that achieve an optimal trade-off between theseobjectives.

Various example embodiments found that red-and-blue QR codes providedvery poor grayscale contrast and were difficult to read. To improve thevisibility of the QR code for scanning under broadband white lightillumination, the colours yellow (with a high average transmittance of48% over the range of 450 nm to 650 nm, which includes most of the powerof a typical white light source) and blue (low average transmittance of22%) were selected for the light and dark pixels, respectively. Thisselection of colours maximises the grayscale image contrast in whitelight while still retaining wavelength selectivity for the multiplexedholograms under monochromatic red and blue light (e.g., see FIG. 6B).

Having selected suitable colours, exploiting the spatial degree offreedom in the multiplexed holograms allows the ability to arrange thepixels into a print that shows a desired colour image, and in thisexample, a meaningful binary image of a QR code 600 (see FIG. 6C)without a significant decrease in the fidelity of the holographicprojections. The QR code 600 stores a 2,620-bit message at errorcorrection level H which can be retrieved by scanning the image with amobile phone or other reader. It can be observed that the holographicprojection switches cleanly between the Chinese seal (see FIG. 6D) andthe Penny Black stamp (see FIG. 6E) under alternating red and blue laserillumination, despite both projections occupying the same region inspace (e.g., see FIG. 18, which will be described later below). Theprint may have a multiscale hierarchical structure (e.g., see FIGS. 6Fto 6H) in which pillar array colour filters of different dimensions andhologram phase plates of varying thickness are seamlessly integrated.

Using the spectral degree of freedom, the constraint on wavelengthselectivity of the colour filters is relaxed according to variousexample embodiments by introducing three additional colours (orange,yellow, and purple) into a three-colour multiplexed RGB (red, green, andblue) hologram and arranging the pixels to form a complex six-colourimage 700 as illustrated in FIG. 7A in greyscale.

FIGS. 7A to 7F relate to an exemplary six-colour holographic colourprint 700 providing enhanced optical security. FIG. 7A depicts atransmission optical micrograph of the colour print 700, a 480×480 pixel(e.g., 1.44 mm²) reproduction of Luigi Russolo's painting “Perfume”under which three holograms have been multiplexed. The ability toinclude colours that are less suitable for multiplexing the holograms(spectral freedom) allowed the ability to expand the usable colourpalette to a total of six colours. FIG. 7B depicts optical micrographsof pillar arrays that produce the colours used in the print 700, namely,added colours orange, yellow, purple (micrographs marked with a dashedbox) and original colours red, green, and blue. Overlaid percentages onthe micrographs of each pillar array denote the proportion of pixels inthe print with that colour. Coloured boxes (as labelled) around thepillar micrographs indicate that they are used in the hologram channelof that colour. FIG. 7C denotes transmission spectra of pillar arraycolour filters used in the print 700, namely, original colours in thinlines; and added colours in thick lines. FIGS. 7D to 7F depictsholographic projections of the print in transmission, photographed on awhite wall in a darkened room, being a red thumbprint (FIG. 7D), a greenkey (FIG. 7E), and blue lettering that reads “SECURITY” (FIG. 7E).Illumination sources were 638 nm red, 527 nm green, and 449 nm bluelasers, respectively. The projection size scales with the projectiondistance, reaching an average size of approximately 10 cm at a distanceof 1 m. Original source images and simulation results of the colourimage and holograms can be found in FIG. 23, which will be describedlater below.

Accordingly, the additional colours were assigned to their closest matchwithin RGB, more specifically, orange colour filters were placed overphase plates belonging to the red hologram and purple colour filtersover phase plates of the blue hologram (see FIG. 7B). As the yellowcolour filters had poor selectivity between transmitting red light andgreen light (FIG. 7C), various example embodiments opted not to storeany hologram information in the phase plates under yellow filters astheir high transmission at both wavelengths would cause the redprojection to appear on the green channel as crosstalk and vice versa.However, if no information was stored in the yellow pixels, variousexample embodiments found that their constant phase would contribute tothe transmitted zero-order (undiffracted) beam. To address this, variousexample embodiments instead applied a random phase to diffuse thecontribution from the yellow pixels into a uniform background.

In the final six-colour print, the high fidelity of holographicprojections and remarkable lack of discernible crosstalk between them(see FIGS. 7D to 7F) demonstrates that it is possible to pattern complexand colourful images without sacrificing the quality of the multiplexedholograms in the same print. Various example embodiments note that thenature of the Perfume print 700 allowed for the use of error diffusiondithering in recolouring the image to obtain an optimal random pixelarrangement for high quality holograms (e.g., see Section “PixelArrangements for Spatial Multiplexing of Holograms”, which will bedescribed below). However, dithering could not be applied to the QR codeprint in FIG. 6A as it locally scrambles the colours and positions ofthe pixels across the entire image, which would render the QR codeimpossible to scan. In order to accurately reproduce the QR code,various example embodiments opted to retain its original (suboptimal)blocky pixel arrangement at the cost of slightly blurring theprojections. The effects of different pixel arrangements on multiplexedholograms are compared in FIG. 22, which will be described later below.

Accordingly, a useful feature of the holographic colour prints accordingto various example embodiments is that it is easy to view both thecolour image and the holographic projections without specialisedequipment (e.g., see Section on “Practical Applicability of HolographicColour Prints”, which will be described later below). The colour imagecan be captured by a handheld phone camera with a macro lens undernarrow-beam white light illumination (e.g., see FIG. 14, which will bedescribed later below) such as that from a collimated flashlight.Because the holographic projections appear on-axis, they are perfectlyoverlapped under collinear illumination and may potentially be used forfull-colour projection (e.g., see FIG. 18, which will be described laterbelow). The projections can be seen over an approximately 30° range ofillumination angles (e.g., see FIG. 15, which will be described laterbelow), which is convenient for handheld viewing with a laser pointer.Due to the relatively high efficiency of the holograms (e.g., see Table2100 in FIG. 21, which will be described later below), the projectionsfrom even a low power laser pointer are visible under normal roomlighting (e.g., see FIG. 17, which will be described later below). Theoverall experimental efficiency of the all-dielectric prints accordingto various example embodiments is as high as 10% to 20% for holographiclaser projection and 30% for colour image transmission (e.g., seeSection on “Efficiency Measurements”, which will be described laterbelow).

Accordingly, various example embodiments developed a monolithicallyintegrated pixel that layers a structural colour filter over a phaseplate to provide combined phase and amplitude control. The design methodor algorithm according to various example embodiments enable printsincluding large arrays of these pixels to be formed or created tosimultaneously fulfil the objectives of hologram multiplexing and colourimage formation. In various example embodiments, holographic colourprints are fabricated as passive standalone devices capable of showing acolour image and multiple holographic projections under differentillumination conditions. Because their phase and amplitude control ispurely structural and the structures are made of a single material, theprints according to various example embodiments can be completelydescribed by a height map, in other words, information may be storedonly in their surface topography. In this regard, various exampleembodiments may replicate the surface relief profile of their structuresby nanoimprint lithography to manufacture the prints. Various exampleembodiments further provides pixel-level control of various propertiesof light for enabling the development of further practical opticalsecurity devices.

An exemplary method 800 for designing or configuring an exampleholographic colour print will now be described with reference to FIG. 8according various example embodiments of the present invention.

FIG. 8 depicts a flowchart of a design (or configuration) method (oralgorithm) 800 for combining colour image formation and spatialmultiplexing of holograms. An initial stage 802 of the method (e.g., afirst step or Step 1) recolours the input colour image using a limitedcolour palette and then divides the colour pixels into several groups(channels) based on their suitability for filtering each hologram. Afterthe assignment in Step 1, the main body or stages of the method (e.g.,second to fifth steps or Steps 2 to 5) may apply a modifiedGerchberg-Saxton algorithm that takes into account the arrangement ofpixels as well as their amplitudes (spectral profiles) and phases inorder to iteratively re-optimise the phase of the holograms on eachchannel. Despite the imperfect selectivity of the pixel amplitudes(e.g., there is non-zero passage of light at unwanted wavelengths, whichresults in crosstalk between channels), a satisfactory balance betweenthe quality of the colour image and holographic projections can beachieved by using a sufficiently large number of pixels, which allowsfor both spatial (e.g., as exemplified by FIGS. 6A to 6H) and spectral(e.g., as exemplified by FIGS. 7A to 7F) degrees of freedom to beexploited.

In various example embodiments, a computer executable program, such asan iterative multi-objective MATLAB code, may be written to perform thecolour image matching and phase calculation for each hologram channel(i.e., the method 800) as shown in FIG. 8. Accordingly, there may beprovided a computer program product, embodied in one or morenon-transitory computer-readable storage mediums, including instructionsexecutable by at least one processor to perform the method 800. Themethod 800 includes obtaining (at 803, e.g., a first step or Step 1) asinput data a set of microscope images and spectral profiles collectedfrom different colour filters (e.g., a large number of different colourfilters) including pillar arrays with varying dimensions (the spectraused were averaged from colour filters on blocks of several thicknesses,as described hereinbefore with reference to FIGS. 5A to 5E). Forexample, this data provides the colour as well astransmittance/reflectance (amplitude) of the pillar arrays at specificdesired wavelengths. At 803, each pixel of the colour image to bepatterned may be colour-matched to the closest available colour in thedataset while balancing two considerations: the majority of the pixelsin the image have colours that are suitable for filtering RGBwavelengths, and the number of unique colours minimised to keeppatterning time short. Once the colour filters are selected, the colourimage is recoloured accordingly and the corresponding spectral profilesare used to generate a map of amplitudes (transmittance/reflectance ofeach pixel at red, green, and blue wavelengths). This information maythen be fed to a modified Gerchberg-Saxton algorithm (e.g., at 805, 807,809, 811, e.g., second to fifth steps or Steps 2 to 5) to iterativelyoptimise the phase of each element so as to best achieve three separatered, green, and blue grayscale holographic projections. In variousexample embodiments, a size of 480×480 pixels may be used for theholographic colour prints, which was found to be computed in less thanone minute by running the iterative multi-objective MATLAB code on amodern laptop (e.g., see “Hologram Computation”, which will be describedlater below).

An exemplary method 900 for fabricating an exemplary holographic colourpixel 901 will now be described with reference to FIGS. 9A and 9Baccording various example embodiments of the present invention.

In the direct laser writing exposure process, as shown in FIG. 9A, a 780nm femtosecond pulsed IR laser beam may be focused into a liquid puddleof negative-tone photoresist 902. At the focal point of the laser spot,the UV-sensitive photoresist may be cross-linked by two-photonpolymerisation and becomes solid. Unexposed photoresist remains as aliquid and may be later washed away during development. The laser spotmay be translated in three dimensions (x, y, and z) according to thesequence as shown in FIG. 9B to create complex structures. Resolution inthe z-direction is not limited by the axially elongated shape of thepoint spread function as it is determined by the positioning accuracy ofthe laser spot.

FIG. 9B depicts a method (process flow) 900 for fabricating aholographic colour pixel 901 based on direct laser writing. The method900 includes creating (at 904, e.g., first step or Step 1) blocks byrastering the laser spot to fill a square in the xy-plane with acontinuous line exposure (hatching); optionally repeating (at 906, e.g.,second step or Step 2) Step 1 at higher z-positions (slicing); creating(at 908, e.g., third step or Step 3) pillar arrays by point exposures,where the diameter may be controlled by the exposure dose; andoptionally repeating (at 910, e.g., fourth step or Step 4) Step 3 athigher z-positions. Block thickness and pillar height may be controlledin Steps 2 and 4 by overlapping multiple layers of exposures along thez-direction. Accordingly, a full holographic colour print may be made bypatterning an array of pixels with various block and pillar dimensions,for example, based on a phase and amplitude map.

In various example embodiments, from the phase and amplitude mapscreated by the design method 800 in FIG. 8, a separate MATLAB codegenerates a blueprint of structures (phase plates and pillar arraycolour filters) with appropriate dimensions to achieve the desired phaseand amplitude. This structural blueprint may be converted into a set ofinstructions for controlling the laser writing sequence used in thefabrication method 900.

Therefore, a holographic colour print including phase plates and colourfilters may be fabricated in a single lithographic process by 3D directlaser writing on glass substrates according to various exampleembodiments. A femtosecond pulsed IR laser may be focused by a highnumerical aperture immersion microscope objective into the photoresistas a tight spot of submicron size. Two-photon absorption andpolymerisation occur in the UV-sensitive photoresist at the focal pointof the laser spot, which can be scanned laterally and shifted axially(refocused) relative to the photoresist/glass interface according to apredefined writing sequence to write a desired pattern including pointsand lines, as illustrated in FIG. 9B. Cross-linking of the negative-tonephotoresist along the laser exposure path creates the phase plates andcolour filters as solid polymer structures on the glass.

In various example embodiments, the area to be patterned was split intoa square grid of 120×120 μm² write-fields based on the maximumundistorted field of view of the microscope objective. The write-fieldswere written sequentially and stitched together by successivetranslations of the stage on which the substrate was mounted. In eachwrite-field, blocks of the same thickness were grouped and the writingsequence in FIG. 9B was performed for each group in ascending order ofthickness. Within each group of blocks, all blocks were patterned beforetheir pillars were patterned. In this regard, grouping the blocks bythickness instead of spatial coordinate has advantageously been found tominimise patterning time, as refocusing in the z-direction was found tobe much slower than lateral scanning in the xy-plane. In various exampleembodiments, the total writing time for a 1.44 mm square print (480×480array of 3 μm pixels) was 6 to 8 hours.

For better understanding of the present invention, according to variousexample embodiments, further details of the holographic colour print andthe method of forming the holographic colour print as describedhereinbefore according to various example embodiments will now bedescribed below, along with various discussions or observations whereappropriate.

Materials

In various example embodiments, in the method of forming the holographiccolour print, such as described hereinbefore with reference to FIGS. 9Aand 9B, solvents were purchased from Sigma-Aldrich and used as-is.Photoresist (IP-dip, Nanoscribe GmbH) and glass substrates (fusedsilica, 25 mm squares with a thickness of 0.7 mm) were purchased fromNanoscribe GmbH.

Sample Fabrication

In various example embodiments, in the method of forming the holographiccolour print, such as described hereinbefore with reference to FIG. 9B,direct laser writing was performed in a Photonic Professional GT system(Nanoscribe GmbH). A 780 nm femtosecond pulsed IR laser with a 90 fspulse duration and 80 MHz repetition rate (Toptica FemtoFiber Pro) wasfocused into a puddle of liquid IP-dip photoresist by an immersionobjective (Zeiss Plan Apo 63×, NA 1.4) to induce two-photon absorptionand polymerisation. The lateral position of the laser spot wascontrolled by using galvanometric mirrors to deflect the beam within thefield of view of the objective lens, whereas the axial position of thespot was controlled by using a piezoelectric and mechanical translationstage to shift the photoresist/substrate interface relative to the focalplane of the objective lens. This rastering of the laser spot createdpixels and prints as cross-linked polymer structures on a glasssubstrate.

The laser power incident on the entrance aperture of the objective lenswas controlled by an acousto-optic modulator (AA Opto-Electronic). Forline exposures (blocks), the scan speed was 8000 μm s⁻¹ and the laserpower 21.0 mW for the first raster scan and 16.8 mW for the secondraster scan. The hatching pitch was 250 nm and slice thickness 0.70 μm.For point exposures (pillars), the exposure time was varied between 0.02ms and 0.04 ms and the laser power between 33.3 mW and 46.4 mW, and theslice thickness ranged from 0.69 μm to 1.01 μm. The slice thickness wasadjusted to match the (dose-dependent) axial elongation of the pointspread function of the laser spot while maintaining a vertical overlapof approximately 30% (300 nm to 430 nm depending on the size of thelaser spot in the vertical direction).

To wash away the excess unexposed liquid photoresist, development wascarried out by immersion of the sample in polyethylene glycol methylether acetate (PGMEA) for 5 minutes and then isopropyl alcohol (IPA) for3 minutes, followed by transfer into nonafluorobutyl methyl ether(NFBME) as a low surface tension solvent for the final drying step. Dueto the large difference in density between the two solvents, residualIPA carried over from the previous step would float on the surface ofNFBME and was siphoned off before removing the sample. This stepminimises recontamination of the sample with IPA when it was withdrawnthrough the surface, as the IPA would otherwise dry on the sample andcause the pillars to collapse due to its relatively high surfacetension.

Phase Plate Thickness Calibration

In various example embodiments, in the method of configuring orcalibrating phase plate thickness, such as described hereinbefore withreference to FIG. 5A, polymerised IP-dip photoresist with a refractiveindex of 1.54 to 1.58 across the visible spectrum was used, and theblock thickness determined for 2π phase modulation was 0.79 μm, 0.95 μm,and 1.17 μm at the design wavelengths of 449, 527, and 638 nm. To spanthe required or desired range of thicknesses and avoid unwanted shiftsin filter colour at thicknesses below 0.6 μm, a thickness range of 0.6μm to 1.8 μm was used. A series of blocks fabricated with differentthicknesses in this range was scanned with a stylus profilometer (KLATencor) at a lateral speed of 10 μm s⁻¹ and a force of 0.10 mg forthickness calibration. Based on the thickness errors found, the limit ofplacement accuracy of the laser spot was estimated to be 100 nm in theaxial direction. As such, the thickness in steps of no smaller than 100nm was discretised, corresponding to quantised phase levels of 0.25π,0.21π, and 0.17π for blue, green and red light, respectively. With astrict lower limit of 100 nm on the thickness step size, the number ofphase levels used in the final prints was rounded down to 7, 9, and 11,respectively. Various example embodiments found that patterning too manyphase levels is time-consuming and can be counterproductive as anunfavourable sequence of axial positioning errors from the piezoelectricstage may then cause undesirable reversals in the phase profile.

Scanning Electron Microscopy

In various example embodiments, scanning electron micrographs wereacquired in a field emission scanning electron microscope (JEOLJSM-7600F) at an accelerating voltage of 5.0 kV and a working distanceof 6.7 mm.

Hologram Computation

In various example embodiments, the iterative multi-objective codedescribed hereinbefore (MATLAB R2017b) was executed on a MicrosoftSurface Pro with an Intel i5-7300 2.60 GHz processor and 8 GB of RAM. Inthis regard, it was found that the design algorithm describedhereinbefore according to various example embodiments generated a480×480 pixel three-colour multiplexed hologram in less than one minute.

Simulations

In various example embodiments, the holographic projections shown inFIGS. 22A to 22C and 23 (which will be described later below) weresimulated based on the output of the design algorithm (a phase map andan amplitude map) using the same source images as in FIG. 4, FIGS. 6C to6E, and FIGS. 7A and 7D to 7F described hereinbefore. In various exampleembodiments, the simulated projections were calculated from the phaseand amplitude maps in MATLAB as follows. First, the element-wise productof the phase map (in phasor form) and the amplitude map is computed,approximating the initial electric field distribution of lightimmediately after being transmitted through the print. Then a Fouriertransform is taken, approximating the propagation of light into the farfield (in the Fraunhofer limit). Lastly, the square modulus is taken toconvert the electric field strength into an intensity image, shown inlogarithmic scale to approximate the human visual response.

Photography of Holographic Projections

In various example embodiments, holograms were projected in transmissiononto a white wall and photographed using a DSLR camera in a darkenedroom. Coherent illumination was provided by 638 nm red, 527 nm green,and 449 nm blue laser diode modules with a maximum power of 4.5 mW(ThorLabs). Within the setup, the actual power at the sample wasmeasured to be approximately 2 mW on average. The distance of theholographic colour prints from the wall (projection distance) was 135cm, at which the holographic projections measured between 10 and 15 cmacross. Photographs of the holographic projections in FIGS. 6D and 6Eand FIGS. 7D to 7F described hereinbefore show an approximately 20 cm²region centered on the projection.

Optical Characterization

In various example embodiments, optical micrographs and spectra wereacquired in a Nikon Eclipse LV100ND optical microscope equipped with aCRAIC 508 PV microspectrophotometer and a Nikon DS-Ri2 camera. Sampleswere backlit by halogen lamp illumination and measured in transmissionthrough a 5×/0.15 NA objective lens. As the colour filters arediffractive in nature, wavelengths that pass are transmitted on-axiswhile rejected wavelengths are diverted off-axis. Thus, thetransmittance spectra were measured in a narrow cone of acceptanceangles using an objective with a numerical aperture of 0.15 (ahalf-angle of 8.6°).

Colour Filter Selection

In various example embodiments, spectra were analysed by comparing theiraverage transmittance within three narrow wavelength bands centered atthe red, green, and blue laser wavelengths, T _(R), T _(G), and T _(B).For each spectrum, these values were used to calculate figures of meritχ_(R), χ_(G), and χ_(B) that determine the suitability of thecorresponding pillar array for use as a red, green, or blue colourfilter. In various example embodiments, the figure of merit for a redcolour filter χ_(R) may be the sum of the difference between red andgreen transmittances and the difference between red and bluetransmittances, i.e., χ_(R)=(T _(R)−T _(G))+(T _(R)−T _(B))=2T _(R)−T_(G)−T _(B).

A matrix representation was used in the code to enable vectorisation ofthe actual calculations.

Although a more accurate figure of merit may take into account thewavelength selectivity for each possible set of filters as a group (aswill be further described later) rather than for individual filters,various example embodiments did not adopt this in the design algorithmas it would greatly increase the number of computations.

Colour Palette and Wavelength Selectivity

According to various example embodiments, ranges of colours attainableby varying the pillar dimensions (e.g., height and diameter) of thepillar array colour filters are shown in FIGS. 10A to 10C. Inparticular, colour palettes derived from pillar array colour filters byvarying the pillar dimensions of height and diameter are shown, forwithout blocks underneath (i.e. with a block thickness of 0 μm) (FIG.10A), with blocks of thickness 1.0 μm under the pillars (FIG. 10B), andwith blocks of random thicknesses in the range 0.6 μm to 1.8 μm underthe pillars (FIG. 10C). The pitch of the pillar arrays is 1.0 μm in allcases.

It was found that pillars on blocks (FIGS. 10B and 10C) give darkercolours that are spectrally shifted from the colours of pillarspatterned directly on the glass substrate (FIG. 10A). Accordingly, invarious example embodiments, the block thicknesses for the prints wereconfigured or selected to lie in a range (0.6 μm to 1.8 μm) over whichthe pillars showed little to no colour change with block thickness(e.g., see FIG. 10C), so as to afford relatively thickness-independentcolour filters. The spectra measured for colour filters with blocks ofthickness in this range were then averaged to minimise the effects ofany remaining thickness dependence on designing prints.

FIGS. 11A and 11B show comparisons of filter colours under differentconditions of block thickness. In particular, FIG. 11A illustrates acolour space plot for colour filters on blocks of thicknesses 0.6, 1.0,1.4 and 1.8 μm, with colours converted from thickness-averaged spectrafor a 1931 CIE 2° standard observer viewing under illuminant D65 (whitelight with a colour temperature of 6500 K). For these colour filters,the pillar height and diameter were varied in the ranges 0.5 μm to 2.7μm and 310 nm to 390 nm, respectively, achieving a 53% coverage of thesRGB colour gamut. FIG. 11B illustrates a colour space plot for colourfilters on blocks of thickness 1.0 μm with no averaging performed. Thereare no major differences between the colour space plots in FIGS. 11A and11B as the colour is almost independent of block thickness in thisthickness range (0.6 μm to 1.8 μm).

Accordingly, the similarity between the colour space plots in FIG. 11A(averaged) and FIG. 11B (1.0 μm thick) show that there was in factalmost no thickness dependence remaining even before averaging. Thecoverage relative to sRGB of the averaged spectra was 53%, wide enoughfor us to pick out suitable filters for colour prints. Table 1200 inFIG. 12 lists the dimensions of pillars and the thickness-averaged RGBtransmittances of the colour filters in the Perfume print 700. Inparticular, Table 1200 shows the dimensions of pillars and RGBtransmittance values for the colour filters in the Perfume print 700.The transmittance spectra were averaged over blocks of thicknesses 0.6,1.0, 1.4 and 1.8 μm, and the transmittance values further averaged overa narrow bandwidth of 4 nm centred at the wavelengths 449 nm (blue), 527nm (green), and 638 nm (red), as well as a broadband spectral range of450-650 nm (white).

By performing experiments to explore the parameter space of pillardimensions, various example embodiments were able to identify a set ofthree colour filters with adequate wavelength selectivity formultiplexing RGB holograms. In various example embodiments, thewavelength selectivity of transmission may be defined as the ratio oftransmittances at the design wavelength for a colour filter designed topass it and a colour filter designed to reject it. The RGB wavelengthselectivity for the chosen or selected set of colour filters wascalculated and shown in Table 1300 in FIG. 13, and ranges from 3.2 to6.6 among the red, green and blue colour filters, for an averageselectivity of 4.6. In particular, Table 1300 shows the RGB wavelengthselectivity of the colour filters in the Perfume print 700. Theselectivity values were calculated from the transmittance values andshown in Table 1200. Mutual selectivities among the red, green and bluecolour filters are highlighted in bold while those involving the yellowfilters (not used for multiplexing) are greyed out.

Balancing Wavelength Selectivity in Multiplexed Holograms

According to various example embodiments, in a multiplexed hologram, theoverall transmission efficiency for a given channel may be the productof the area fraction occupied by the channel and the weighted average ofthe transmittance of the colour filters on that channel, with an upperbound of 33% for the case of equal area fractions in an RGB hologram. Ifunequal area allocation arises from a predominance of one or two coloursin the colour image to be printed, this can be compensated by adjustingthe colour balance of the image before colour matching. Alternatively,it can also be desirable to deliberately encourage an unequal areaallocation when the wavelength selectivity of the filters on one channelis significantly worse than those on others. In this manner, the numberof the total hologram pixels allocated to each channel can be adjustedto balance out the transmission characteristics of the filters. Forexample, if the desired green transmission of the green filters (signal)does not sufficiently exceed the unwanted green transmission of the redand blue filters (noise, which manifests as crosstalk), more greenpixels can be allocated to increase the signal-to-noise ratio on thegreen channel.

According to various example embodiments, a useful metric is the signalstrength, which may be defined for each channel as the product of itsarea fraction and its average transmittance at its design wavelength.Noise strength terms may be defined analogously as the product of thearea fraction of a channel and its average transmittance at the designwavelengths of other channels. In this regard, various exampleembodiments construct a matrix with the signal strengths on the diagonaland the noise strengths as off-diagonal (cross) terms, where thesignal-to-noise contrast across all channels is balanced when it mostclosely approximates a diagonal matrix with a constant baseline shift.

Using this signal-to-noise matrix, it was found that the appearance ofcrosstalk in the holographic projections was minimised by applying aslight green tint to the colour balance of the source image for thePerfume print 700 in FIG. 7A to give a pixel allocation of 36% in thegreen channel, 29% in the red channel, 27% in the blue channel, and 8%of yellow pixels not assigned to any channel. This green tint is notobvious in the final printed image, but is only used in the design stageto promote a desired colour matching outcome.

Practical Applicability of Holographic Colour Prints

According to various example embodiments, for practical application ofthe holographic colour prints as optical security devices, the printsare configured to be usable under non-ideal conditions and without theaid of a specialised viewing setup. Accordingly, in various exampleembodiments, the colour images are configured so as to not require amicroscope to be seen, and the holographic projections are robust todeviations in the illumination angle and easily viewed with a standardlaser pointer even in the presence of ambient background light.

FIGS. 14A and 14B show structural colour images viewed under differentnumerical aperture conditions. In particular, FIG. 14A depicts aphotograph of the QR code print taken using a handheld phone camera(APPLE iPhone 8 Plus) mounted with a low cost 10× macro lens attachment(Shuohu). The sample was backlit with white light transmissionillumination from a microscope condenser set to a small numericalaperture of 0.1 for optimal viewing. Any white light source with anarrow beam angle may also be used for illumination. The inset shows theoptical micrograph from FIG. 6C for comparison. FIG. 14B shows acomparison of pillar array colour filters imaged with a 50×/0.40 NAmicroscope objective (left image of FIG. 14B) and a 50×/0.80 NAmicroscope objective (right image of FIG. 14B) with the condenser set toa matching illumination numerical aperture. In various exampleembodiments, the illumination numerical aperture may be limited to below0.4 so that the large collection numerical aperture of a macro lens orhigh magnification objective does not wash out the diffractive colour.

Accordingly, FIG. 14A demonstrates the possibility of viewing theholographic colour prints according to various example embodiments usinga handheld phone camera with a low cost macro lens attachment. The QRcode colour image can be clearly seen although fine details cannot beresolved as the macro lens used did not have a high enoughmagnification. Various example embodiments note that while illuminationfrom a microscope condenser was used for convenience, a focusable orcollimated flashlight may also be used to illuminate the colour printsas long as it produces a sufficiently narrow beam. In various exampleembodiments, a narrow beam may be required or preferred because of thediffractive nature of the colour filters, as explained in the following.

The colour filters may separate incident light into two components: onethat is transmitted on-axis (the desired colour) and another that isdiffracted away from the optical axis (the complementary colour, whichis unwanted). For the intended subtractive colour effect to be produced,only the desired colour is collected according to various exampleembodiments. If the collection angle of the lens is too large, bothcomponents are collected and the colour becomes washed out as theyrecombine to give the colour of the light source. Alternatively, ifillumination is delivered over too wide a range of angles, the angularseparation between the two components is lost and again both arecollected. Thus the range of illumination and collection angles, i.e.,the combined numerical aperture of the imaging system, may be consideredaccording to various example embodiments when attempting to view ourcolour prints. Since the collection numerical aperture is set by themacro lens, various example embodiments instead control the illuminationnumerical aperture. Based on the numerical aperture dependence observedin FIG. 14B, the diffractive colour can still be seen up to a numericalaperture of 0.4, or a beam angle of up to ±23°, which is achievable bymany common commercially available flashlights.

FIG. 15 shows Chinese seal holographic projection viewed at variousillumination angles, according to various example embodiments of thepresent invention. Illumination angles are specified in the top-rightcorner of each image, with 0° being normal incidence. Positive(negative) angles represent clockwise (anticlockwise) rotations of theprint, where the print was rotated to the right (left) while the laserand screen were not adjusted. The projection was maintained over anapproximately 30° range of illumination angles, only fading away at −15°to −10° on the left and 15° to 20° on the right.

In particular, to investigate the angle dependence of our prints underlaser illumination, the angle at which the beam was incident on a printwas varied and the holographic projections were photographed as before.FIG. 15 shows the result of this test on the Chinese seal projection ofthe QR code print. The projection is essentially angle-insensitivebetween −5° and 10°, and suffers a slight decrease in brightness at −10°and 15° with little loss in quality. The projection is faintly visibleat −15° and 20° and disappears as the angle is increased farther. Theslight asymmetry in the usable range of illumination angles might be dueto a small average tilt in the pillars of 2° to 3° relative to thenormal, possibly introduced during the drying step of the developmentprocess. Due to the approximately 30° range of illumination angletolerance, it is easy to project the holograms by holding the print inone hand and a laser pointer in the other.

FIG. 16 illustrates the wide-angle characteristics of the Chinese sealholographic projection. Photographs were taken at different projectiondistances d, with the sample placed: (left image) near a white sheet ofpaper (d=20 cm) and (right image) far from a white wall (d=135 cm). Thecentral projection (pointed by arrow 1604) is surrounded by higherdiffraction orders of the projection (pointed by arrows 1606). At higherangles, the Penny Black stamp holographic projection from the otherchannel (pointed by arrows 1608) can be seen. The undesirable featuresthat disturb the central projection at d=20 cm are much less apparent atd=135 cm, at which the projection has expanded from 2 cm to 12.5 cm, thehigher orders have faded, and the unwanted projection is now over 70 cmaway from the main projection. Thin grey lines 1610 mark boundarieswhere the screen makes a right angle (between two sheets of paper onleft, and between two walls on right). The schematic (bottom)illustrates the workings of the colour filters and holograms: red lightis passed (indicated by thick arrows 1620) by the yellow colour filters1622 and rejected (indicated by thin arrows 1624) by the blue colourfilters 1626, creating a diffraction pattern in the far field upon whichthe holographic projections are superimposed. Because the diffractionangle is inversely related to the pitch, the hologram phase plates(pixel pitch 3 μm, scale bar 1628) only weakly modulate the angles ofthe incident light as compared to the colour filters (pillar pitch 1 μm,scale bar 1630), which divert the light far off-axis when they rejectit. As a result, the unwanted projections are well separated from themain projection at 3× the angular separation from the center (indicatedby thin arrows 1632) as compared to the first-order peaks (indicated bythick arrow 1634). Note that the projections photographed at d=20 cm andd=135 cm are both well within the far field.

Accordingly, FIG. 16 illustrates the wide-angle characteristics of theprint, comparing the appearance of the projection at different distancesfrom the print to the screen. At a distance of 20 cm, the projectionsize of 2 cm is too small to avoid significant contamination with thecentral undiffracted zero-order spot. Other undesirable features arealso present: the projection is repeated at higher diffraction ordersaway from the center, and at even higher angles, the projection from theother channel and its higher diffraction orders can be seen. Asexplained above, because the colour filters work by diffracting unwantedwavelengths off-axis, the unwanted projections still appear, but only atvery high angles. Various example embodiments address these problems byusing a longer projection distance on the order of 1 m or farther. At adistance of 135 cm, the projection has expanded to a size of 12.5 cm andits features can be more clearly discerned even in the presence of thezero-order spot. The higher diffraction orders (which contain much lesspower) are weak enough that they can barely be seen, and the unwantedprojections from the other channel are far off to the sides, over 70 cmaway. Thus, various example embodiments use a projection distance of 135cm and show photographs of only approximately 20 cm² including thecentral projection in FIGS. 6D and 7D.

Various example embodiments note that the relative angle independence ofthe projections and the repeating of the projections at higher ordersare both characteristic of holograms in the so-called “thin hologram”regime, which applies to the holograms according to various exampleembodiments as their thickness of 0.6 μm to 1.8 μm was smaller thantheir pixel pitch of 3 μm. This is unlike the case of “thick” volumeBragg gratings for which the thickness is much larger than the pitch,which results in sharply angle-dependent projections in only the zerodiffraction order.

Various example embodiments note that although the projections werephotographed in a darkened room for clarity, this is not a requirementfor viewing the holograms. The laser power of 2 mW that was usedafforded bright projections that could be clearly seen under standardroom lighting conditions (e.g., see FIG. 17). In particular, FIG. 17shows the ease of viewing the Chinese seal holographic projection underambient lighting. The projection is bright and clear in the presence ofstrong background light in the room despite using only 2 mW of laserpower. Projected on a white wall 135 cm away, the projection measures12.5 cm across. Similar results were obtained using common commerciallyavailable Class 3R laser pointers with an output power of up to 5 mW.The large size of the projections also makes for convenient viewing ofthe holograms.

Because of the on-axis nature of the projections, the projected imagesoverlap perfectly as long as their illumination sources are collimatedand collinear (beams sharing the same axis). For example, colour mixingof the red and blue lasers to give purple occurs in the overlap of theprojections. This demonstrates the ability of the method according tovarious example embodiments to achieve multi-colour projection and showfull colour holograms. In particular, FIG. 18 illustrates simultaneoustwo-colour holographic projection from the QR code print. Underillumination with collinear red and blue lasers, the side profile ofQueen Victoria (white dashed outline) in the centre of the Penny Blackstamp (blue) overlaps with the Chinese seal (red) and so appearsmagenta, demonstrating the possibility of achieving multi-colourprojection. The projection distance is 135 cm.

Measuring Amplitude-Phase Coupling

As described hereinbefore, phase-amplitude coupling was minimised usinga range of phase plate thicknesses that produced only minor variationsin the colour of the pillar colour filters. Hence, various exampleembodiments have experimentally shown that phase variation would notgreatly affect the amplitude. However, amplitude-phase coupling maystill be present, i.e., the pillar colour filters may contribute anadditional phase shift on top of that imparted by the underlying phaseplates, such that control of amplitude also affects the phase. If thisunwanted additional phase shift is significant and uncompensated, itcould disrupt the holographic projections in multi-colour prints such asthe Perfume print 700, which have more than one colour in each colourchannel. Amplitude-phase coupling would not affect the QR code printbecause it imposes a uniform phase shift on each colour channel, whichhas no effect on the projections.

To quantify any phase shift caused by the pillar colour filters, variousexample embodiments fabricated and compared binary phase gratings withphase elements including either: (1) phase plate blocks of two differentthicknesses, or (2) two sets of pillars with different dimensionsarrayed on top of a base layer of blocks of uniform thickness. The phaseplate grating (1) uses blocks of 1.0 and 1.5 μm thickness, which werechosen to produce relative phase shifts of approximately 0 and π acrossthe visible spectrum. For the pillar array gratings (2), pillars withsignificantly different dimensions were configured to maximise theirphase difference, but with similar transmittances at the designwavelength so as to achieve a relatively flat amplitude profile acrossthe grating. Under these conditions, any diffraction observed would bedue primarily to a periodic phase variation created by the phasedifference between the two sets of pillar arrays. Then comparing thepower in the diffraction orders of gratings (1) and (2) allows us todirectly compare the strength of phase modulation by blocks and pillararrays. FIG. 19 shows the fabricated gratings and their dimensionsaccording to various example embodiments.

In particular, FIG. 19 shows the binary phase gratings made of phaseplates and pillars according to various example embodiments.Transmission optical micrographs of checkerboard binary phase gratingscomposed of: (top image) phase plates of thickness 1.0 and 1.5 μm, and(bottom images, starting from left) red, green and blue pillars,respectively, imaged with a 10×/0.20 NA objective. The inset was imagedwith a 100×/0.90 NA objective. Schematics of the two grating types areshown on far left. The full checkerboards have 240×240 squares, whereeach square is a 2×2 super-pixel of holographic colour pixels (3×3pillar array on top of a 3×3 μm² block) as shown in the inset in FIG.19. In the pillar array gratings, the blocks form a constant 1.0 μmthick base layer while the squares alternate between two slightlydifferent colours (different pillar dimensions) that have a similartransmittance at the design wavelength.

It was observed that the power diffracted into the first order by thephase plate grating was more than ten times of that diffracted by thepillar array phase grating (e.g., see FIG. 20). This result shows thatthe colour filters may affect the phase of the transmitted light, buttheir effect is an order of magnitude smaller than that of the phaseplate blocks. As such, it is reasonable to neglect amplitude-phasecoupling in the design of holographic colour prints according to variousexample embodiments. Various example embodiments note that fullcharacterisation of the phase imparted would allow straightforwardcorrection of this coupling in the design stage by changing theunderlying block thickness in each pixel to compensate for the extraphase, which could potentially improve the quality of the projections bya factor of about 10%.

In particular, FIG. 20 depicts a comparison of the diffracted power forphase plate gratings and pillar array gratings. As labelled in thetop-left schematic, the power in the zero order is directly measuredfrom the central bright spot (filled circle) while the power in thefirst order is summed over the four diffraction peaks (open circles)produced by the checkerboard gratings. The photographs of thediffraction patterns under red laser illumination show zero order spotsof similar intensity but much stronger first order peaks for the phaseplate grating, as confirmed by the measurements in the Table shown inFIG. 20. Misalignment errors during stitching of the write-fields inthese prints introduce an additional periodicity along the x- andy-directions, which superimposes a horizontal and vertical flare on allthe diffraction peaks. The flare does not substantially alter theresults of the experiment as it reduces the power in each peak by anequal percentage. The same phase plate grating is measured with allthree lasers (638 nm red, 527 nm green and 449 nm blue) while eachpillar array grating is measured with the laser of the correspondingcolour.

Efficiency Measurements

In various example embodiments, the transmission and diffractionefficiency of the QR code print was calculated and shown Table 2100 inFIG. 21. For simplicity, various example embodiments ignore thediffractive nature of the pillar colour filters and treat them astransmissive elements—the unwanted projections diffracted off-axis areconsidered to be rejected, while the on-axis central projection ispassed (e.g., see FIG. 16). In this manner, various example embodimentsmay define the filter transmission efficiency on each channel as theratio of the power in the central projection (transmitted through theprint) to the power transmitted through bare glass at the designwavelength. Having accounted for the transmission characteristics,various example embodiments then define the hologram diffractionefficiency as the ratio of diffracted power (power in the centralprojection but not in the zero-order spot) to the power in the centralprojection.

The filter transmission efficiency of the QR code sample was measuredusing a power meter placed immediately before and after the sample. Thetransmission efficiency was measured to be 32% for blue and 34% for redlaser illumination, close to the expected value of 34% for a about 50%area fraction of pixels with a transmittance of 68% (blue pixels at bluelaser wavelength and yellow pixels at red laser wavelength). Thehologram diffraction efficiency is 72% for the blue projection and 47%for the red projection. The overall efficiency of the print, calculatedas the product of the transmission efficiency of the glass substrate,the filter transmission efficiency, and the hologram diffractionefficiency, is 21% for blue and 14% for red. These efficiencies aresufficient for the holographic projections from the prints according tovarious example embodiments to be visible at low laser power and in abright environment as for example seen in FIG. 17. Further improvementsin the hologram efficiency and overall efficiency may be made if futureadvances in the axial positioning accuracy of the laser writer allow thephase plate thickness profile of the holograms to more closelyapproximate their ideal phase profiles (e.g., see Section on “PhasePlate Thickness Calibration” as described hereinbefore).

Various example embodiments also calculated the on-axis transmissionefficiency of the Perfume print 700 in FIG. 7A for showing a colourimage under white light illumination. Taking the weighted average of thefilter transmittances for white light (see Table 1200 in FIG. 12) withthe proportion of pixels of the corresponding colours (e.g., see FIG. 7)resulted in an efficiency of 30%.

Accordingly, Table 2100 in FIG. 21 shows the efficiency of the QR codeprint according to various example embodiments. A power meter was usedto collect the light in the central projection (including the zero-orderspot) some distance from the print (“power in central projection”) andsolely within the zero-order spot at a farther distance (“power inzero-order spot”). In various example embodiments, further measurementswere made, first replacing the print with an unpatterned glass substrate(“transmitted power”) and then removing the print altogether (“incidentpower”). Power is given in units of mW.

Pixel Arrangements for Spatial Multiplexing of Holograms

Holographic colour prints lie on a continuum between colour images, inwhich the arrangement of pixels is rigidly defined, and multiplexedholograms, for which the arrangement of pixels is seemingly arbitrary.However, even if the requirement to form a colour image is removed,there are still restrictions on the types of pixel arrangements that canbe used for hologram multiplexing. Because the holograms according tovarious example embodiments are Fraunhofer holograms that operate in theFourier domain, the Fourier transform of the (real space) pixelarrangement enters into the determination of the final holographicprojections—specifically, the final holographic projection is thespatial convolution of the designed holographic projection with theFourier transform of the pixel arrangement. According to various exampleembodiments, the implications of this mathematical relationship on thedesign of multiplexed holograms are elaborated on in the following.

Adopting an idealised matrix representation of the pixel arrangement,the presence or absence of a hologram pixel at each location in space isdenoted respectively by an amplitude of one or zero in the correspondingposition of a 2D matrix (a binary mask). Then a matrix of onescorresponds to a hologram that completely fills the illuminated area anddiverts the entire incident beam to project a desired image, whereas amatrix of zeroes corresponds to an illuminated area unoccupied byhologram pixels such that the incident beam passes straight through andremains as a spot.

In the space-division wavelength-multiplexing scheme according tovarious example embodiments, the pixels of each hologram only occupypart of the total area, which gives a “patchy” pixel arrangement on eachwavelength channel. When pixels are removed from a complete,unmultiplexed hologram to create a patchy pixel arrangement (introducingzeroes into a matrix of ones), the undiffracted central (zero-order)bright spot increases in intensity at the expense of the projectedimage. While the projected image might then simply be expected to fadeaway gradually as pixels are removed, it can in fact become blurred orrepeated. This is because the holographic projection is not onlyaffected by the number of pixels remaining, but is also highly sensitiveto the locations of the remaining pixels (i.e. the pixel arrangement).

For better understanding, consider that the Fourier transform of aconstant amplitude profile (the pixel arrangement of a complete,unmultiplexed hologram) is a Dirac delta function and returns anidentical projection after convolution. However, the Fourier transformof the pixel arrangement of a patchy hologram is a combination of aDirac delta and some noise terms which draw power away from the Diracdelta. Convolution with such a “noisy delta” function may have theeffect of creating unwanted copies of the holographic projection (“ghostimages”) that weaken and distract from the desired central projection.

These disturbances to the holographic projection can become especiallypronounced when the Fourier transform has localised regions of highintensity noise that concentrate the ghost images and make them moreapparent—which may be a particularly serious issue when imperfectwavelength selectivity causes them to appear as crosstalk on multiplexedchannels. In general, any form of ordering or periodicity in a pixelarrangement may be manifested as clustering or peaks in its Fouriertransform and thereby accentuate the crosstalk noise in multiplexedholographic projections, as shown in the simulated far field projectionsin FIGS. 22A to 22C.

In particular, FIGS. 22A to 22C illustrate the effect of pixelarrangement on multiplexed holograms. A comparison of 480×480 red,green, and blue (RGB) pixel arrangements for hologram multiplexing andsimulated far field holographic projections, based on the RGB laserwavelengths and transmission spectra in FIG. 5B. Above each projectionis the pixel arrangement for its colour channel, rescaled to show itskey features (left image), and the Fourier transform of the pixelarrangement (right image). FIG. 22A illustrates an example random pixelarrangement in which individual RGB pixels are interspersed to give afeatureless appearance. FIG. 22B illustrates an example “blocky” randompixel arrangement in which randomness is only applied down to a scale of40-pixel blocks. FIG. 22C illustrates an example periodic pixelarrangement in which a 4×4 super-pixel is tiled to fill the space.According to various example embodiments, the random arrangement shownin FIG. 22A may be the most suitable for hologram multiplexing as itaccurately reproduces the source images with minimal crosstalk. With theblocky arrangement shown in FIG. 22B, unwanted “ghost images” areconcentrated around the central holographic projection, forming adiffuse glow that highlights the crosstalk in the background. In theperiodic arrangement shown in FIG. 22C, the ghost images are repeatedacross the Fourier plane at locations determined by the positions ofpeaks in the Fourier transform of the pixel arrangement.

Compared with a random pixel arrangement which produces threeprojections with little crosstalk (e.g., see FIG. 22A), a “blocky” pixelarrangement band-limits the Fourier power spectrum, which concentratesghost images around the central projection and gives it a blurryappearance (e.g., see FIG. 22B). Meanwhile, a periodic pixel arrangementcreates regular peaks in the power spectrum, which causes the tiling ofghost images in the Fourier plane (e.g., see FIG. 22C). The crosstalk,which is barely noticeable in FIG. 22A, becomes much more apparent inFIGS. 22B and 22C as the overlapping of ghost images enhances theirvisibility not only in their own channels but also on other channels.

Based on the above analysis, the optimal pixel arrangements formultiplexing according to various example embodiments are those whichcan spread out the ghost images by diffusing the noise power uniformlyacross the entire frequency domain to create a flat power spectrum, orequivalently, by generating a white noise signal in real space. Variousexample embodiments found that a convenient way to achieve a white noisespectrum was to use an error diffusion dithering algorithm to performthe necessary colour matching between the colour image to be printed andthe colour palette available.

Various example embodiments use the Floyd-Steinberg error diffusionalgorithm implemented in MATLAB as the built-in function dither. Ascompared to naively mapping a colour image to a more limited colourpalette by directly applying error minimisation to each pixel, ditheringdiffuses the quantisation error of each pixel over its neighbouringpixels so as to spread out the error uniformly. Doing so improves theappearance of high-error regions with little degradation of low-errorregions, thereby increasing the apparent quality of the recolouredimage.

Apart from increasing the perceived colour accuracy beyond the resultsof simple error minimisation algorithms (which improves the quality ofthe colour print), dithering also minimises occurrence of largesingle-colour blocks of pixels, typically breaking them up into acomplicated halftone pattern of various other colours. This scramblingof the colour pixels helps to randomise the pixel arrangement on eachcolour channel and generate a flatter power spectrum more similar tothat of white noise (which improves the fidelity of holographicprojections).

FIG. 23 shows the original source images 2302 and simulations 2304according to various example embodiments as described hereinbefore. Inparticular, source images 2302 and corresponding simulations 2304 of thePerfume colour print 700 and holographic projections are shown.Simulations are based on the RGB laser wavelengths and colour filtertransmission spectra in FIG. 5B and the pixel layouts from FIGS. 6C and7A. The colour print was simulated for a 1931 CIE 2° standard observerviewing under illuminant D65 (white light with a colour temperature of6500 K).

Accordingly, various example embodiments are in the domain of optics andphotonics, as the holographic optical element may be an optical orphotonic component. Various example embodiments may provide ananostructured surface obtained through nanofabrication processes, andsuch a physical embodiment may have applications in optical documentsecurity. In various example embodiments, the fields of nanofabricationand security may also be relevant.

Conventional phase holograms may project a single image when illuminatedwith a laser. Furthermore when viewed by eye, the plane of the hologramitself appears featureless because it contains phase information and notamplitude information. In contrast, in various example embodiments, byincorporating structural or other color filters onto the hologram, animage on the plane of the hologram may be produced, instead of afeatureless patch. Furthermore, when illuminated with lasers ofdifferent colors, multiple grayscale images may be selected forholographic projection.

Various example embodiments provide a single optical element thatsimultaneously allows for amplitude control of transmitted white lightto show a colour image and enables phase control of laser light to showseveral different holographic projections. Accordingly, wavelengthmultiplexing of holograms “hidden” under a colour print may beperformed, which may serve as a multi-level security feature.

For example, a (microscopic) colour print of an image may be integratedwith a hologram by patterning the colour image directly on top of thehologram. Thus the colour image is seen when viewing under white light,but a holographic projection is revealed upon illumination with laserlight. By careful design (e.g., as described hereinbefore according tovarious example embodiments), more than one hologram may be incorporatedinto the same colour print. Various example embodiments use the colourprint as a colour filter layer to provide wavelength selectivity forviewing of one or more different holograms. Accordingly, various exampleembodiments provide a wavelength-multiplexing scheme or technique forholograms, where several holograms are combined by dividing the spaceinto regions of arbitrary shapes and sizes onto which colour filters arepatterned. When the multiplexed hologram is illuminated, the colourfilters select the appropriate projection by allowing or hindering thepassage of the different wavelengths of light. The colour filter layercan also be designed to show a separate colour image, independent of theinformation encoded in the holograms. Accordingly, various exampleembodiments may be applicable to both transmission and reflectionholograms, and may be most easily implemented using computer-generatedholography to calculate or determine the holograms andmicro/nano-fabrication techniques for creating the physical form of theholograms.

In various example embodiments, because the encoded information isdistributed throughout all parts of a hologram, even an incomplete copyof a hologram (for example a random subset of the hologram) can producethe desired holographic projection, albeit with some errors and loss ofresolution. Exploiting this robustness to missing parts, hologramsdesigned for use at different wavelengths can be patterned within thesame area by removing selected regions of each hologram to preventoverlaps between the spatially co-located holograms. In various exampleembodiments, colour filters are fabricated on the respective regionsassigned to each hologram, so that each region allows passage of theappropriate wavelength of light for its hologram and hinders the passageof the others. In this way, illumination of the entire hologram witheach wavelength of light will project only the correct grayscale imageof each colour (from the appropriate regions) while the other unwantedimages (arising from illumination of regions designed for otherwavelengths) are either not produced or discarded.

In various example embodiments, the holograms may be considered to bewavelength-multiplexed into different colour channels, as each hologramencodes an independent set of information which can be accessed andseparated based on the illumination wavelength. For example, themultiplexed holograms fit together like coloured jigsaw puzzle pieces,which may be arranged in any random or aperiodic fashion (to avoidsuperimposing an additional periodicity onto the holographicprojections). By choosing to assign the spatial regions for each colourchannel in a specific way, the physical hologram itself can be made inthe shape of a colour image which is independent of the encoded holograminformation. In various example embodiments, the colour range of theimage can be improved by allowing more variation in the colour filtersused, at the expense of the wavelength selectivity of the grayscaleholographic projections.

In various example embodiments and in practice, a reasonable balancebetween colour range and wavelength selectivity can be achieved suchthat two or more grayscale holograms can be encoded into an unrelatedcolour image, creating an unusual and unique micro-print that may beuseful in security applications.

Accordingly, various example embodiments have successfully demonstratedthe encoding of two different grayscale holograms into a holographiccolour microprint of a completely unrelated colour image. As mentionedhereinbefore, while various example embodiments have been described withrespect to transmission holograms, it will be appreciated to a personskilled in the art that the present invention is also applicable toreflection holograms and is within the scope of the present invention.

In various example embodiments, the holographic colour microprint may beunderstood as a stack of two independent optical elements, one as aphase hologram and the other as an amplitude image. This allows for twodegrees of freedom at each position in the holographic microprint,namely, blocks control the phase in a wavelength-dependent manneraccording to their thickness, while pillars control the amplitude in awavelength-dependent manner according to their transmission orreflection spectrum.

In various example embodiments, for illustration purposes to demonstratean example design or configuration of the holographic optical elementand without limitation, dielectric nano-pillars are used as transmissioncolour filters and dielectric nano-blocks are used as transmission phaseplates, as described hereinbefore. In this regard, an array ofnano-pillars with varying height and diameter may serve as a colourprint, and an array of nano-blocks with varying thickness may serve as acomputer-generated hologram. In various example embodiments, by layeringor disposing the nano-pillar array directly on top of the nano-blockarray, the desired outcome of colour image formation and hologrammultiplexing can be achieved.

In various example embodiments, together, the nano-pillars andnano-blocks may form a monolithic structure that can be fabricated in asingle step process by 3D direct laser writing in a suitablenegative-tone photoresist, as described hereinbefore. For example, the2.5D nature of the structure suggests that it is possible to massproduce by a nano-imprinting process.

In various example embodiments, the dimensions of nano-pillars: heightmay be in a range from 0.5 to 3.0 μm; and diameter may be in a rangefrom 100 to 500 nm. The dimensions of nano-blocks: thickness may be in arange from 50 nm to 2000 nm; lateral size (e.g., width and length) maybe in a range from 1 to 10 μm.

Various example embodiments note that although the pillars may impart aphase shift to the light and thereby introduce errors in the hologramphase, this may be compensated for by applying an appropriate correctionto the thickness of each underlying block, which may be variedindependently of the pillars. Alternatively, the height and diameter ofthe pillars may be varied to compensate for shifts in the pillar colourdue to the presence of underlying blocks. In various exampleembodiments, a balance between the two types of compensation may besought in order to achieve optimal performance.

In various example embodiments, it was found that the performance of theexperimentally demonstrated holographic colour microprint is reasonablygood even without applying these sorts of compensation. The colour imageand hidden holograms are also relatively robust to fabrication errorssuch as slanting pillars and distorted blocks.

In various example embodiments, one full-colour image and multiplegrayscale images may be simultaneously encoded into a single holographiccolour microprint. The microprint itself shows the full-colour imagewhen illuminated with white light, and can be easily seen under amagnifying glass. Meanwhile, illumination with laser light at eachhologram design wavelength projects a corresponding grayscale image thatis viewable by eye when cast on any suitable surface such as a wall orfloor. The holographic projections are naturally parallel and coaxial,and are thus perfectly aligned at any distance without the need tointroduce any additional physical components or design modifications.

In various example embodiments, the holograms are relativelyangle-insensitive unlike those in the angular multiplexing (object planespace-division) method, for which the holograms require a fixed readoutgeometry with specific illumination angles for each colour, and mayrequire the use of a microscope to view the projections.

In various example embodiments, the laser light sources do not need tobe positioned on specific areas of the hologram, and broad illuminationacross the hologram is sufficient.

Accordingly, the holographic optical element (e.g., microprints)according to various example embodiments may be used as security printsfor anti-counterfeiting applications, for example, on bank notes orimportant documents to provide optical security. For example, while thecolour print may possibly be duplicated by high-resolution colourprinting, the underlying “hidden” holograms are very difficult orimpossible to replicate with any standard printing process unless amaster of the holographic microprint is available. This allows for anadditional degree of security as compared to conventional colourmicroprints.

Accordingly, various example embodiments advantageously allow the designrequirements of several holograms and a colour image to be decoupled byseparating the holograms and the colour elements into different layers(or different portions). Various example embodiments enable thedifferent layers (e.g., the two layers) to be constructed as a singlemonolithically integrated holographic microprint, whereby themultiplexed holograms are intimately intermixed throughout the space (orarea) of the hologram layer (or hologram portion).

While embodiments of the present invention have been particularly shownand described with reference to specific embodiments, it should beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the scope of thepresent invention as defined by the appended claims. The scope of thepresent invention is thus indicated by the appended claims and allchanges which come within the meaning and range of equivalency of theclaims are therefore intended to be embraced.

What is claimed is:
 1. A holographic optical element comprising: ahologram portion comprising a plurality of groups of unit regions, eachgroup of unit regions of the hologram portion being configured toproduce a respective holographic image under a respective lightillumination having a respective predetermined wavelength; and a colourfilter portion formed on the hologram portion, the colour filter portioncomprising a plurality of groups of unit regions, each group of unitregions of the colour filter portion being arranged on a correspondinggroup of the plurality of groups of unit regions of the hologramportion, wherein the plurality of groups of unit regions of the colourfilter portion is spatially arranged to form a predetermined colourimage.
 2. The holographic optical element according to claim 1, whereinat least one of the groups of unit regions is interspersed amongst oneor more other groups of the plurality of groups of unit regions.
 3. Theholographic optical element according to claim 1, wherein each group ofunit regions of the colour filter portion is configured with wavelengthselectivity for the light illumination associated with the correspondinggroup of the plurality of groups of unit regions of the hologram portionand against at least one of one or more light illuminations associatedwith one or more remaining groups, respectively, of the plurality ofgroups of unit regions of the hologram portion.
 4. The holographicoptical element according to claim 3, wherein each group of unit regionsof the colour filter portion is configured with wavelength selectivityfor the light illumination associated with the corresponding group ofthe plurality of groups of unit regions of the hologram portion andagainst each of the one or more light illuminations associated with theone or more remaining groups, respectively, of the plurality of groupsof unit regions of the hologram portion.
 5. The holographic opticalelement according to claim 1, wherein the plurality of groups of unitregions of the hologram portion is spatially arranged to correspond tothe spatial arrangement of the plurality of groups of unit regions ofthe colour filter portion forming the predetermined colour image, andeach group of the plurality of groups of unit regions of the hologramportion is configured to produce the respective holographic image basedon the spatial arrangement of the group of unit regions of the hologramportion.
 6. The holographic optical element according to claim 5,wherein each unit region of the group of unit regions of the hologramportion comprises a thickness respectively configured for modifying aphase of the light illumination associated therewith such that the groupof unit regions of the hologram portion collectively produce therespective holographic image under the light illumination.
 7. Theholographic optical element according to claim 6, wherein the thicknessis in a range of 0.6 μm to 1.8 μm.
 8. The holographic optical elementaccording to claim 1, wherein each unit region of the group of unitregions of the colour filter portion comprises a spectral profilerespectively configured to allow passage of the light illuminationassociated with the corresponding group of the plurality of groups ofunit regions of the hologram portion and to hinder passage of each ofthe one or more light illuminations associated with the one or moreremaining groups, respectively, of the plurality of groups of unitregions of the hologram portion.
 9. The holographic optical elementaccording to claim 1, wherein each unit region of the group of unitregions of the colour filter comprises an array of pillar structures.10. The holographic optical element according to claim 1, wherein thehologram portion and the colour filter portion are made of a dielectricmaterial.
 11. The holographic optical element according to claim 1,wherein the hologram portion and the colour filter portion are formed asa monolithic structure.
 12. The holographic optical element according toclaim 1, wherein the light illuminations associated with the pluralityof groups of unit regions of the hologram portion, respectively, arelaser illuminations and are different from each other.
 13. Theholographic optical element according to claim 12, wherein the laserilluminations associated with the plurality of groups of unit regions ofthe hologram portion are each selected from a group consisting of a redlaser illumination, a green laser illumination and a blue laserillumination.
 14. A method of forming a holographic optical element, themethod comprising: forming a hologram portion comprising a plurality ofgroups of unit regions, each group of unit regions of the hologramportion being configured to produce a respective holographic image undera respective light illumination having a respective predeterminedwavelength; and forming a colour filter portion on the hologram portion,the colour filter portion comprising a plurality of groups of unitregions, each group of unit regions of the colour filter portion beingarranged on a corresponding group of the plurality of groups of unitregions of the hologram portion, wherein said forming the colour filterportion comprises spatially arranging the plurality of groups of unitregions of the colour filter portion to form a predetermined colourimage.
 15. The method according to claim 14, wherein said forming thecolour filter portion further comprises interspersing at least one ofthe groups of unit regions amongst one or more other groups of theplurality of groups of unit regions.
 16. The method according to claim14, wherein said forming the colour filter portion further comprisesconfiguring each group of unit regions of the colour filter portion withwavelength selectivity for the light illumination associated with thecorresponding group of the plurality of groups of unit regions of thehologram portion and against at least one of one or more lightilluminations associated with one or more remaining groups,respectively, of the plurality of groups of unit regions of the hologramportion.
 17. The method according to claim 16, wherein each group ofunit regions of the colour filter portion is configured with wavelengthselectivity for the light illumination associated with the correspondinggroup of the plurality of groups of unit regions of the hologram portionand against each of the one or more light illuminations associated withthe one or more remaining groups, respectively, of the plurality ofgroups of unit regions of the hologram portion.
 18. The method accordingto claim 14, wherein said forming the hologram portion furthercomprises: spatially arranging the plurality of groups of unit regionsof the hologram portion to correspond to the spatial arrangement of theplurality of groups of unit regions of the colour filter portion formingthe predetermined colour image; and configuring each group of theplurality of groups of unit regions of the hologram portion to producethe respective holographic image based on the spatial arrangement of thegroup of unit regions of the hologram portion.
 19. The method accordingto claim 18, wherein said forming the hologram portion furthercomprises: configuring respectively each unit region of the group ofunit regions of the hologram portion to have a thickness for modifying aphase of the light illumination associated therewith such that the groupof unit regions of the hologram portion collectively produce therespective holographic image under the light illumination. 20.(canceled)
 21. The method according to claim 14, wherein said formingthe colour filter portion further comprises: configuring respectivelyeach unit region of the group of unit regions of the colour filterportion to have a spectral profile for allowing passage of the lightillumination associated with the corresponding group of the plurality ofgroups of unit regions of the hologram portion and for hindering passageof each of the one or more light illuminations associated with the oneor more remaining groups, respectively, of the plurality of groups ofunit regions of the hologram portion.
 22. (canceled)
 23. (canceled) 24.(canceled)
 25. (canceled)
 26. (canceled)
 27. An article having opticalsecurity incorporated therein, the article comprising: a substrate; andone or more holographic optical elements formed on the substrate forproviding the optical security, each of the one or more holographicoptical elements comprising: a hologram portion comprising a pluralityof groups of unit regions, each group of unit regions of the hologramportion being configured to produce a respective holographic image undera respective light illumination having a respective predeterminedwavelength; and a colour filter portion formed on the hologram portion,the colour filter portion comprising a plurality of groups of unitregions, each group of unit regions of the colour filter portion beingarranged on a corresponding group of the plurality of groups of unitregions of the hologram portion, wherein the plurality of groups of unitregions of the colour filter portion is spatially arranged to form apredetermined colour image.